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Nest Thermostat Not Cooling: Complete Troubleshooting Guide

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When your Nest thermostat fails to activate your air conditioning system, the problem leaves you uncomfortable during hot weather and requires systematic troubleshooting to identify whether the issue lies with the thermostat, wiring, or HVAC equipment.

Nest “not cooling” problems affect all Nest models -Learning Thermostat, Thermostat E, and the 2020 Nest Thermostat – but most causes are straightforward to diagnose and fix.

Understanding Nest Cooling Operation

Your Nest thermostat doesn’t cool your home directly—it sends low-voltage (24V) signals through wires to your air conditioning system, instructing it when to start and stop. When you set a cooling temperature, Nest closes an electrical circuit that powers your AC’s control systems, which then activate the compressor, condenser fan, and indoor blower.

Critical concept: “Nest not cooling” problems fall into three categories: (1) Nest isn’t sending the cooling signal at all, (2) Nest sends the signal but wiring prevents it from reaching the AC, or (3) the AC receives the signal but doesn’t respond due to equipment problems. Effective troubleshooting requires determining which category applies.

Common Causes of Nest Not Cooling

1. Incorrect Thermostat Settings or Configuration

Sometimes Nest appears to not be cooling when actually it’s just configured incorrectly or settings prevent cooling operation.

Common configuration problems:

Heating mode instead of cooling:

  • Nest set to “Heat” mode instead of “Cool” or “Heat•Cool”
  • In Heat mode, lowering temperature won’t activate cooling
  • Display shows orange/red (heating) instead of blue (cooling)

Temperature setpoint above current temperature:

  • Cooling setpoint accidentally set higher than current room temperature
  • AC won’t run until temperature rises above setpoint
  • Common after someone raises setpoint thinking it will cool faster

Schedule conflicts:

  • Scheduled temperature change overriding manual adjustment
  • Eco Temperature active and preventing cooling to desired setpoint
  • Home/Away settings keeping temperature in energy-saving range

Safety temperature enabled:

  • Safety Temperature feature limiting how cold Nest will go
  • Found in Settings → Safety Temperature
  • May prevent cooling below certain threshold (e.g., 65°F)

Symptoms:

  • Nest display shows cooling icon but nothing happens
  • Temperature setpoint seems unresponsive
  • Display shows “DELAYED” or “IN 2+ HR” messages
  • Cooling worked previously but stopped after schedule change

Solution:

Check mode setting:

  1. Press Nest display to wake
  2. Note display color (blue for cooling, orange for heating, black for off)
  3. Turn outer ring to change mode to Cool or Heat•Cool
  4. Verify blue cooling icon appears when temperature set below current

Verify temperature setpoint:

  1. Check current temperature displayed in center
  2. Confirm cooling setpoint is at least 2-3°F below current temperature
  3. Turn ring counterclockwise to lower setpoint significantly
  4. Look for blue cooling icon and “COOLING” status

Check schedule and settings:

  1. Go to Settings → Schedule
  2. Review programmed temperatures for current time
  3. Disable Auto-Schedule temporarily: Settings → Nest Sense → Auto-Schedule → Off
  4. Set manual temperature and verify cooling activates
  5. Check Eco Temperatures: Settings → Eco → adjust if too restrictive

Disable Safety Temperature limits:

  1. Settings → Safety Temperature
  2. Check if minimum cooling temperature set too high
  3. Disable or adjust to allow desired cooling setpoint
  4. Test cooling at lower temperature

2. Power Issues Preventing Cooling Signal

Nest thermostats require adequate electrical power to send cooling signals reliably. Power problems often affect cooling specifically because AC systems draw more current than heating.

Power-related cooling failures:

  • Insufficient power without C-wire causes Nest to prioritize basic functions
  • Low battery prevents high-current cooling signal
  • Weak transformer can’t supply current for AC contactor coil
  • Power stealing through Y-wire (cooling) causes voltage drop

Symptoms:

  • Nest displays “Low battery” or “Check wiring” warnings
  • Display dims when attempting to cool
  • Cooling works intermittently or for short periods
  • Technical Info → Power shows Voc below 24V or battery below 3.6V
  • Heating works but cooling doesn’t (heating often requires less power)

Testing procedure:

Check power status:

  1. Press Nest display to wake
  2. Go to Settings → Technical Info → Power
  3. Note these values:
    • Voc (voltage open circuit): Should be 24-28V AC
    • Vin (input voltage): Should stay above 20V
    • Battery: Should be 3.7-3.9V when healthy
  4. If Voc below 24V, power supply inadequate
  5. If battery below 3.6V, charging problem exists

Test cooling with adequate power:

  1. Manually run cooling for 15-20 minutes
  2. Check if Nest battery voltage increases
  3. If cooling works initially but stops, power insufficient for sustained operation
  4. If cooling never activates, power likely not the issue

Solution:

Install C-wire (most reliable):

  • Check for unused wire in thermostat bundle (often blue)
  • If wire exists, connect to C terminal at both Nest and HVAC system
  • Provides continuous 24V power independent of HVAC operation
  • Cost: $0 if wire exists; $150-400 for professional installation

Install Nest Power Connector:

  • C-wire adapter included with Nest Thermostat E
  • Available separately for other Nest models (~$15-25)
  • Installs at HVAC system, requires single wire back to Nest
  • Professional installation recommended: $100-200

Upgrade HVAC transformer:

  • Replace undersized transformer (20-30VA) with 40-50VA+ unit
  • Necessary if multiple HVAC components draw power
  • Provides adequate current for reliable cooling operation
  • Professional installation required: $150-300

Temporary workaround (not recommended long-term):

  • Charge Nest by running fan continuously for several hours
  • Monitor battery voltage in Technical Info
  • This doesn’t solve root problem; cooling may still fail

3. Y-Wire (Cooling Wire) Problems

The Y-wire (typically yellow) carries the cooling signal from Nest to your air conditioner’s contactor. Y-wire problems are extremely common in “not cooling” scenarios.

Y-wire issues:

Not connected or loose:

  • Wire disconnected at Nest base or HVAC system
  • Terminal screw not tightened adequately
  • Wire pulled out during installation or maintenance

Connected to wrong terminal:

  • Y-wire connected to W (heating), G (fan), or other terminal
  • Multiple wires on single terminal causing confusion
  • Wiring diagram not followed correctly

Damaged or broken:

  • Wire cut, frayed, or damaged in wall cavity
  • Corrosion at wire ends preventing electrical contact
  • Wire insulation compromised causing short circuits

Missing jumper in some systems:

  • Some heat pump systems require jumper between Y and other terminals
  • Dual-fuel systems may need specific Y-wire configuration
  • Two-stage cooling systems need Y1 and Y2 properly connected

Symptoms:

  • Nest shows “COOLING” status but AC doesn’t run
  • Blue cooling icon appears but no equipment activates
  • Fan runs but compressor/condenser don’t start
  • Error message: “No power to Y wire” or similar

Testing Y-wire:

Visual inspection:

  1. Turn off HVAC power at breaker
  2. Remove Nest display from base
  3. Verify yellow wire connected to Y terminal
  4. Tighten terminal screw if loose
  5. Check wire condition (no fraying, corrosion, or damage)
  6. Ensure sufficient bare wire exposed (1/4 inch minimum)

Continuity test:

  1. Keep HVAC power off
  2. Disconnect Y-wire at both Nest and HVAC system
  3. Use multimeter set to continuity/resistance mode
  4. Test Y-wire end-to-end (should show near 0 ohms resistance)
  5. Test Y-wire to other wires (should show infinite resistance/no continuity)
  6. If continuity exists to other wires, short circuit present

Check HVAC system wiring:

  1. Locate control board at furnace or air handler
  2. Verify Y-wire connected to correct terminal (usually labeled “Y” or “Y1”)
  3. Check for loose connections at control board
  4. Verify no visible wire damage near HVAC equipment

Solution:

Reconnect or tighten Y-wire:

  • Strip wire back 1/4 inch if corroded
  • Insert firmly into Y terminal
  • Tighten terminal screw securely
  • Gently tug wire to verify solid connection
  • Repeat at HVAC system control board

Replace damaged Y-wire:

  • If wire damaged in wall, replace entire wire run
  • Use 18-gauge thermostat wire
  • Run new wire from Nest to HVAC system
  • Label clearly at both ends
  • Professional installation recommended: $150-350

Correct wiring configuration:

  • Consult old thermostat wiring photos
  • Verify Nest configuration matches actual system type
  • Check heat pump systems have O/B wire configured correctly
  • Add necessary jumpers for dual-fuel or two-stage systems

4. Incorrect HVAC System Configuration in Nest

Nest’s software needs to know what type of HVAC system you have to send correct signals. Misconfiguration causes cooling failures even with proper wiring.

Configuration problems:

Wrong equipment type selected:

  • Configured as “Forced air” when actually heat pump
  • Selected “Heat pump” when actually conventional AC + furnace
  • Wrong fuel type selected (gas vs. electric vs. oil)

O/B wire orientation incorrect (heat pumps):

  • Heat pumps use O or B wire to switch between heating and cooling
  • Some brands energize on cooling, others on heating
  • Wrong setting prevents cooling operation entirely

Stage configuration incorrect:

  • Single-stage system configured as two-stage
  • Two-stage system configured as single-stage
  • Emergency heat settings incorrect

C-wire configuration wrong:

  • Nest thinks C-wire exists when it doesn’t (or vice versa)
  • Causes power delivery issues affecting cooling

Symptoms:

  • Cooling worked with old thermostat but not with Nest
  • Heating works perfectly but cooling doesn’t
  • Configuration process skipped or rushed during installation
  • Equipment activates but wrong components run

Solution:

Reconfigure equipment type:

  1. Go to Settings → Equipment
  2. Follow setup wizard completely
  3. Answer all questions accurately:
    • What kind of system? (select correct type)
    • Wires detected: R, G, Y, W, C (verify matches actual)
    • For heat pumps: Does your system have O or B wire?
    • Heat pump orientation: Cooling (most common) or Heating
  4. Complete all configuration screens
  5. Test cooling after reconfiguration

Verify heat pump settings:

  1. Settings → Equipment → Continue → Check orientation
  2. Most brands use “Energize on Cool”: Carrier, Trane, American Standard, Goodman, Amana, Rheem, Ruud
  3. Some brands use “Energize on Heat”: Older units, some regional brands
  4. If cooling doesn’t work, try opposite setting
  5. Consult HVAC equipment manual for correct specification

Check stage configuration:

  1. Settings → Equipment → Continue through setup
  2. Note if Nest detected Y1, Y2 (two-stage cooling)
  3. Verify matches actual equipment capability
  4. Single-stage: only Y wire
  5. Two-stage: Y1 and Y2 wires (or Y and Y2)

5. Air Conditioning Equipment Failure

Sometimes Nest sends the cooling signal correctly but the AC equipment doesn’t respond due to mechanical or electrical failure.

Common AC equipment problems:

Tripped circuit breaker or blown fuse:

  • Outdoor condenser unit breaker tripped
  • Indoor air handler breaker tripped
  • Fuse at disconnect box blown
  • Prevents AC from receiving power regardless of thermostat signal

Failed contactor:

  • Contactor receives 24V signal from Nest but doesn’t close
  • Burned contacts prevent current flow to compressor
  • Contactor coil failed (doesn’t energize when signaled)
  • Visible pitting, burning, or damage on contactor

Low refrigerant:

  • Refrigerant leak causes low pressure
  • Low-pressure safety switch prevents compressor operation
  • System had cooling capacity decline over time before complete failure

Dirty or frozen evaporator coil:

  • Clogged air filter restricts airflow
  • Evaporator coil freezes into block of ice
  • Safety switches prevent operation until ice melts

Failed compressor or capacitor:

  • Compressor motor failed (won’t start even with signal)
  • Run capacitor or start capacitor failed
  • Compressor makes humming sound but doesn’t start

Safety switch or sensor failure:

  • High-pressure switch tripped
  • Low-pressure switch prevents operation
  • Condensate overflow safety switch engaged
  • Float switch in drain pan activated

Symptoms:

  • Nest shows “COOLING” and blue icon but no AC operation
  • Fan runs but compressor/outdoor unit doesn’t start
  • AC worked previously but gradually declined
  • Outdoor condenser fan runs but compressor doesn’t
  • Strange noises from AC equipment
  • Ice visible on refrigerant lines or indoor coil

Testing procedure:

Check circuit breakers:

  1. Locate electrical panel/breaker box
  2. Find breakers labeled for AC, air conditioner, or outdoor unit
  3. Check if breaker tripped (handle in middle position)
  4. If tripped, flip fully OFF then back ON
  5. Test cooling after reset
  6. If immediately trips again, electrical short or equipment failure exists

Check outdoor disconnect:

  1. Locate disconnect box at outdoor condenser unit
  2. Open cover (may require screwdriver)
  3. Check cartridge fuses if present (look for broken filament)
  4. Verify disconnect switch in ON position
  5. Close cover and test

Verify outdoor unit runs:

  1. Set Nest to cool, lower temperature well below current
  2. Wait 5 minutes (compressor delay normal)
  3. Go outside and observe condenser unit
  4. Should hear compressor running (loud humming/buzzing)
  5. Condenser fan should spin
  6. Feel air coming from top—should be hot
  7. If nothing runs, equipment problem exists

Check indoor air handler:

  1. Verify fan runs when cooling called
  2. Listen for unusual sounds (grinding, squealing)
  3. Feel supply registers for cold air
  4. Check air filter (replace if dirty)
  5. Look for ice on refrigerant lines near air handler

Solution:

User-fixable issues:

  • Reset tripped breakers
  • Replace blown fuses at disconnect
  • Replace dirty air filter (every 1-3 months)
  • Turn off cooling for 4-6 hours to let ice melt
  • Clear debris from outdoor condenser unit

Professional HVAC service required for:

  • Failed contactors ($100-250 replacement)
  • Refrigerant leaks and recharge ($200-1,500)
  • Failed capacitors ($150-400)
  • Compressor failure ($1,500-3,000)
  • Frozen coils requiring cleaning ($150-400)
  • Safety switch issues ($100-300 diagnosis and repair)
  • Any refrigerant work (requires EPA certification)

6. Nest Delay Features Preventing Immediate Cooling

Nest includes several protective delay features that may make it appear cooling isn’t working when actually it’s just delayed.

Built-in delays:

Compressor protection delay:

  • 5-minute minimum delay between cooling cycles
  • Prevents compressor damage from rapid cycling
  • Display shows “IN 2+ HR” or “DELAYED”
  • Cannot be overridden (safety feature)

Alt heat/emergency heat delays:

  • Heat pump systems delay switching between modes
  • Prevents simultaneous heating and cooling
  • May take 5-10 minutes to switch to cooling after heating

Early-On feature:

  • Nest starts cooling before scheduled time to reach target
  • May appear to not respond if already pre-cooling
  • Display shows “COOLING TO” with target temperature

Temperature swing/differential:

  • Nest allows temperature to rise above setpoint before activating cooling
  • Default swing ~1°F (cooling starts when temp reaches setpoint + 1°F)
  • Reduces cycling frequency, saves energy

Symptoms:

  • Display shows “DELAYED” message
  • Cooling icon present but no equipment running
  • Status shows “IN 2+ HR” or specific time delay
  • Just switched from heating to cooling
  • Cooling eventually starts after waiting

Solution:

Wait for delay to complete:

  • Compressor delays are 5 minutes maximum
  • Mode-change delays typically 5-10 minutes
  • Simply wait; cooling will start automatically
  • Do not adjust settings repeatedly (resets timers)

Check status display:

  • Look for delay messages explaining wait
  • “DELAYED” indicates compressor protection active
  • “IN 2+ HR” indicates cooling scheduled for later
  • Adjust schedule if current comfort unsatisfactory

Adjust temperature significantly:

  • Lower setpoint 5-10°F below current temperature
  • Increases priority and may reduce some delays
  • Will cause longer cooling cycle when it starts

Understanding is key:

  • These delays are intentional equipment protection
  • Overriding would damage expensive AC components
  • Patient waiting is the solution, not a problem to fix

7. Wiring Shorts or Incorrect Common Connection

Electrical shorts between thermostat wires or problems with the common (C) wire connection cause erratic cooling operation.

Short circuit problems:

  • Y-wire insulation damaged, touching other wires
  • Stray wire strands bridging terminals
  • Moisture inside Nest base causing electrical paths
  • Pinched wires in wall cavity

Common wire issues:

  • C-wire connected to wrong terminal at HVAC system
  • Two-transformer systems with improper common reference
  • C-wire connected to equipment ground instead of transformer common
  • No C-wire but Nest configured as if one exists

Symptoms:

  • Cooling activates wrong equipment (fan but no compressor)
  • Multiple functions activate simultaneously
  • Nest randomly reboots when cooling called
  • Error messages about wiring or power
  • Breakers trip when cooling activates

Testing procedure:

Short circuit test:

  1. Turn off HVAC power at breaker
  2. Remove Nest display from base
  3. Disconnect all wires from base
  4. Use multimeter set to continuity mode
  5. Test each wire pair:
    • R to Y: should show infinite resistance (no connection)
    • R to G: should show infinite resistance
    • Y to G: should show infinite resistance
    • Any continuity between signal wires indicates short
  6. Test each wire to ground (metal electrical box if present)

Common wire verification:

  1. At HVAC system, locate transformer
  2. Verify C terminal at Nest connects to transformer common terminal
  3. Common typically marked “C” or “COM” at control board
  4. Should NOT connect to ground terminal or chassis ground
  5. Measure voltage R to C (should be 24-28V AC)

Solution:

Repair shorted wires:

  • Trace wire bundle to find damaged section
  • Replace damaged wire entirely (don’t splice)
  • Ensure wires separated and insulation intact
  • Use wire nuts for any necessary connections
  • Secure wire bundle to prevent future pinching

Correct C-wire connection:

  • Disconnect C-wire at HVAC system
  • Locate proper common terminal on transformer or control board
  • Reconnect C-wire to correct terminal
  • Verify voltage measurements correct
  • Test cooling operation

Professional help needed if:

  • Extensive wiring damage requiring wall opening
  • Two-transformer systems needing proper configuration
  • Electrical panel modifications required
  • Uncertain about proper connections

8. Nest Software Glitches or Firmware Issues

Rarely, software bugs or firmware problems cause cooling failures even with proper hardware and wiring.

Software-related causes:

  • Corrupted firmware after failed update
  • Schedule conflicts or deleted schedules
  • Sensor calibration drift
  • Learning algorithm errors
  • Third-party integration conflicts

Symptoms:

  • Problem started after firmware update
  • Heating works perfectly but cooling doesn’t (unlikely to be hardware)
  • Nest app shows cooling active but thermostat display doesn’t
  • Temperature readings seem inaccurate
  • Recent schedule changes preceded cooling failure

Solution:

Restart Nest thermostat:

  1. Press display to wake
  2. Settings → Reset → Restart
  3. Or: Remove display from base, wait 60 seconds, reattach
  4. Allow Nest to fully boot and reconnect to WiFi
  5. Test cooling after restart

Factory reset (if restart doesn’t work):

  1. Settings → Reset → All Settings
  2. Will erase all schedules and settings
  3. Reconfigure from scratch
  4. Test basic cooling before adding schedules and features
  5. Add features gradually to identify any causing problems

Force firmware update:

  1. Settings → Software → Update
  2. Even if current version shown, check for updates
  3. Allow update to complete without interruption
  4. Test cooling after update

Disable learning features temporarily:

  1. Settings → Nest Sense → Auto-Schedule → Off
  2. Settings → Nest Sense → Home/Away Assist → Off
  3. Set manual temperature schedule
  4. Test if cooling works without automated features

Contact Nest support if:

  • Software reset doesn’t resolve issue
  • Problem clearly software-related not hardware
  • Unit under warranty (2 years for hardware, 1 year for software)
  • Need firmware pushed specifically to your device

Systematic Troubleshooting Process

Step 1: Verify Nest is Actually Calling for Cooling

Before diving into complex troubleshooting, confirm Nest is attempting to activate cooling:

Checklist:

  • [ ] Nest display shows blue color (indicates cooling mode)
  • [ ] Temperature setpoint set at least 2-3°F below current temperature
  • [ ] Display shows “COOLING” status (not “OFF” or “DELAYED”)
  • [ ] Cooling icon (snowflake) visible on display
  • [ ] No delay messages (“IN 2+ HR”, “DELAYED”)

If display shows anything other than active cooling status:

  • Problem is settings/configuration, not equipment
  • Review Section 1 (Incorrect Settings)
  • Adjust mode, temperature, or schedule as needed

If display shows active cooling but nothing happens:

  • Nest sending signal but something preventing cooling
  • Continue to Step 2

Step 2: Check Equipment Status Display

Nest provides diagnostic information showing exactly what it’s telling your HVAC system:

View equipment status:

  1. While cooling is called (display shows “COOLING”)
  2. Press display to wake
  3. Go to Settings → Equipment
  4. Look for “Current Status” or similar screen
  5. Note which components Nest is activating:
    • Y or Y1: Cooling (compressor should run)
    • G: Fan (indoor blower should run)
    • OB: Heat pump reversing valve (if applicable)

Interpretation:

  • If Y showing active, Nest sending cooling signal correctly
  • If Y not showing active, configuration problem exists
  • If Y active but equipment not running, wiring or AC problem

Step 3: Perform Manual HVAC Test (Bypass Nest)

Directly activating your AC bypasses the Nest to determine if problem is thermostat or equipment:

WARNING: This involves working with low-voltage wiring. Turn off HVAC power at breaker first for safety.

Bypass test procedure:

  1. Turn off HVAC power at main breaker
  2. Remove Nest display from base
  3. Disconnect all wires from base
  4. Identify R-wire (red, power) and Y-wire (yellow, cooling)
  5. Turn HVAC power back on at breaker
  6. Touch R-wire and Y-wire together for 3-5 seconds
  7. AC equipment should activate (compressor and outdoor unit run)
  8. Separate wires to stop
  9. Turn power off, reconnect wires to Nest base properly

Results:

If AC runs when wires touched:

  • AC equipment functional
  • Nest or wiring preventing signal from reaching equipment
  • Problem is thermostat, wiring, or configuration

If AC doesn’t run when wires touched:

  • AC equipment problem (not Nest)
  • Proceed to equipment troubleshooting (Section 5)
  • Professional HVAC service likely needed

If unsure or uncomfortable performing test:

  • Skip to professional diagnosis
  • Risk of electrical shock exists if done incorrectly
  • May void warranties if damage occurs

Step 4: Check Power and Technical Information

Review Nest’s diagnostic screens for power and system information:

Power diagnostics:

  1. Settings → Technical Info → Power
  2. Note these readings:
    • Voc: Should be 24-28V AC (if lower, power supply weak)
    • Vin: Should maintain 20V+ during operation
    • Battery: Should be 3.7-3.9V (below 3.6V indicates charging problem)
    • Current: Shows power draw (higher when cooling)

Wiring information:

  1. Settings → Equipment → Continue
  2. Review detected wires
  3. Verify Y-wire shows as connected
  4. Check for any error messages about wiring

Version information:

  1. Settings → Technical Info → Software
  2. Note current version
  3. Check for available updates

Step 5: Inspect All Wiring Connections

Physical inspection often reveals loose or damaged connections:

At Nest base:

  1. Turn off HVAC power at breaker
  2. Remove Nest display
  3. Take clear photo of wiring before touching
  4. Check each wire:
    • [ ] Terminal screw tight
    • [ ] 1/4 inch bare wire exposed
    • [ ] No fraying or corrosion
    • [ ] Wire fully inserted in terminal
    • [ ] No loose strands touching other terminals
  5. Tighten all connections
  6. Look for any wires not connected (might be C-wire)

At HVAC system:

  1. Locate furnace or air handler
  2. Remove control board access panel
  3. Identify where Nest wires connect
  4. Check all connections tight
  5. Verify Y-wire connected to Y or Y1 terminal
  6. Look for damaged wire near furnace

Step 6: Test Cooling in Different Scenarios

Systematic testing helps identify specific conditions causing failure:

Test scenarios:

1. Manual mode test:

  • Turn off all schedules and automation
  • Manually set to Cool mode
  • Lower temperature well below current
  • Wait 5+ minutes
  • Note if cooling activates

2. Different temperature test:

  • Try extreme setpoint (e.g., 60°F)
  • Increases priority and urgency
  • Some configurations activate at larger differentials

3. Time-of-day test:

  • Test cooling at different times
  • May reveal schedule-related issues
  • Morning vs. evening tests

4. With/without fan test:

  • Test with fan set to Auto
  • Test with fan set to On
  • Fan issues sometimes prevent cooling activation

5. After restart test:

  • Restart Nest (Settings → Reset → Restart)
  • Test cooling immediately after reboot
  • Software glitches sometimes cleared temporarily

Document results:

  • Note which scenarios work and which don’t
  • Patterns help identify root cause
  • Provide information to professionals if needed

Solutions Summary by Symptom

“COOLING” Shows But Nothing Happens

Most likely causes:

  1. Y-wire not connected or damaged (60%)
  2. AC equipment failure—breaker, contactor, refrigerant (25%)
  3. Incorrect heat pump O/B wire configuration (10%)
  4. Short circuit in wiring (5%)

First steps:

  • Check Y-wire connections at Nest and HVAC
  • Verify outdoor breaker and disconnect
  • Perform manual bypass test (touch R to Y)

Display Shows “DELAYED” or “IN 2+ HR”

Most likely causes:

  1. Compressor protection delay (5 minutes)
  2. Scheduled temperature change not yet reached
  3. Mode switch delay (heat pump switching from heat)

First steps:

  • Wait 5-10 minutes for delay to complete
  • Check schedule for programmed temperature changes
  • Verify mode is Cool or Heat•Cool (not Heat)

Fan Runs But Compressor Doesn’t

Most likely causes:

  1. Y-wire problem (not sending compressor signal)
  2. Failed contactor at outdoor unit
  3. Tripped breaker at outdoor disconnect
  4. Low refrigerant or safety switch

First steps:

  • Verify Y-wire connected and tight
  • Check outdoor unit breaker/disconnect
  • Observe if outdoor condenser fan runs
  • Professional HVAC assessment for refrigerant/compressor

Nest Keeps Restarting When Cooling Starts

Most likely causes:

  1. Insufficient power (no C-wire)
  2. Weak transformer can’t supply cooling current
  3. Short circuit drawing excessive current

First steps:

  • Check for C-wire or install Nest Power Connector
  • Measure voltage (Settings → Technical Info → Power)
  • Test for short circuits between wires

Works Sometimes, Not Others

Most likely causes:

  1. Loose wire connection (intermittent contact)
  2. Schedule or temperature differential settings
  3. Power supply marginal (works when cool, fails when hot)
  4. AC equipment overheating or safety switches

First steps:

  • Tighten all wire connections
  • Disable Auto-Schedule and test manually
  • Check power readings during failure vs. success
  • Monitor AC equipment for cycling issues

When to Call a Professional

Call HVAC Technician If:

Immediate service needed:

  • AC makes loud grinding, squealing, or banging sounds
  • Burning smell from AC equipment
  • Ice forming on refrigerant lines or outdoor unit
  • Water leaking from indoor unit
  • Breaker trips repeatedly when cooling attempted

Professional assessment recommended:

  • Bypass test shows AC equipment doesn’t respond
  • Outdoor unit doesn’t run at all
  • Compressor hums but doesn’t start
  • Gradual cooling capacity decline over time
  • Refrigerant leak suspected (ice, reduced cooling, hissing sounds)

Expected costs:

  • Diagnostic service call: $75-150
  • Contactor replacement: $100-250
  • Capacitor replacement: $150-400
  • Refrigerant recharge: $200-800
  • Compressor replacement: $1,500-3,000
  • Full AC replacement: $3,000-7,000

Call Nest/Thermostat Professional If:

Professional thermostat help needed:

  • All wiring appears correct but cooling still doesn’t work
  • Need C-wire run through finished walls
  • Complex system configuration (zoned, multi-stage, dual-fuel)
  • Uncomfortable working with wiring or unsure of connections

Expected costs:

  • Thermostat installation/troubleshooting: $100-200
  • C-wire installation: $150-400
  • System reconfiguration: $100-200

Contact Nest Support If:

Google Nest support recommended:

  • Software issues suspected (works sometimes, not others randomly)
  • Factory reset doesn’t resolve problem
  • Thermostat under warranty (2 years hardware, 1 year software support)
  • Need firmware update or remote diagnostics

Contact options:

  • support.google.com/googlenest (help articles)
  • Phone support available for warranty issues
  • Chat support through Nest app
  • Community forums for user experiences

Preventive Measures

During Nest installation:

  • Always install C-wire when possible (prevents 70% of problems)
  • Take detailed photos of old thermostat wiring before removal
  • Carefully follow Nest setup wizard completely
  • Test both heating and cooling immediately after installation
  • Verify equipment type configuration matches actual system

Ongoing maintenance:

  • Replace HVAC air filter every 1-3 months
  • Annual professional AC maintenance (clean coils, check refrigerant)
  • Check wire connections every 6-12 months (tighten if loose)
  • Update Nest firmware when available
  • Monitor power readings monthly (Settings → Technical Info → Power)
  • Test cooling operation monthly, even during winter

Seasonal preparation:

  • Before summer: Test cooling when temperature first rises
  • Check outdoor unit clear of debris, plants, grass clippings
  • Verify indoor vents open and unobstructed
  • Test cooling at various setpoints to ensure responsiveness

Early warning signs to address:

  • Cooling takes longer to reach temperature
  • Temperature swings wider than before
  • Short cycling (cooling runs <10 minutes per cycle)
  • Strange sounds from AC equipment
  • Rising energy bills without usage increase
  • Ice forming anywhere on AC system

Conclusion

Nest thermostat cooling failures typically fall into three categories: configuration errors, wiring problems, or AC equipment issues. Start troubleshooting by verifying Nest is actually calling for cooling—check that display shows blue color, “COOLING” status, and temperature set below current reading. Many “not cooling” problems are actually just incorrect mode settings or schedule conflicts.

If Nest shows active cooling but nothing happens, the Y-wire (yellow cooling wire) is the most common culprit. Check that it’s connected firmly at both Nest and your HVAC system, with no damage or corrosion. A simple manual bypass test—touching the R-wire and Y-wire together—instantly reveals whether the problem is the thermostat/wiring or the AC equipment itself.

Power delivery issues cause many cooling failures, particularly in systems without a C-wire. Installing a C-wire or Nest Power Connector resolves most power-related cooling problems and costs $20-400 depending on whether you do it yourself or hire a professional. This investment prevents repeated cooling failures and ensures reliable operation.

When wiring and power are adequate but cooling still fails, AC equipment problems – failed contactors, low refrigerant, tripped breakers, or frozen coils -require professional HVAC diagnosis and repair. These equipment issues aren’t Nest’s fault, but Nest helps you identify them by showing it’s sending the cooling signal that equipment isn’t answering. Systematic troubleshooting following this guide helps you resolve simple issues yourself and provides valuable information for professionals when needed.