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Fix a heat pump that will not cool

We'll confirm cooling mode, rule out thermostat and filter, then isolate the cause—reversing valve, refrigerant, or compressor—or tell you when to call a pro.

Troubleshooting · Home appliances
15–30 min
What you'll need
  • Replacement air filter (if dirty)
  • Garden hose or brush (for outdoor coil)
Step 1 of 8

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Steps

Goal: Confirm cooling mode, rule out simple causes, then isolate the fault.

  • Check the thermostat. It must be set to “Cool” or “AC” and the temperature below room temperature.
  • Good: Thermostat in Cool mode. Proceed to Check thermostat and filter.
  • Bad: Thermostat in Heat or Auto—set to Cool and retest.

Check thermostat and filter

Goal: Rule out thermostat setting, power, and filter before assuming a mechanical fault.

  • Check the circuit breaker and disconnect switch. Replace the air filter if dirty.
  • Confirm the outdoor unit runs when the thermostat calls for cooling. You should hear the compressor and fan.
  • Good: Power on, filter clean, outdoor unit runs. Proceed to Outdoor unit not running or No cool air based on symptoms.
  • Bad: Breaker tripped or filter very dirty—fix and retest.

Outdoor unit not running

Goal: Get the outdoor unit to run when cooling is requested.

  • If the outdoor unit does not run, the contactor, capacitor, or thermostat wiring may have failed. Check that power is present at the disconnect.
  • Clean the outdoor coil—debris blocks heat transfer and can cause the unit to shut down.
  • Good: Unit runs after power or coil fix.
  • Bad: Unit still does not run—call a technician. The contactor or capacitor may need replacement.

No cool air

Goal: Isolate reversing valve or refrigerant issues when the unit runs but does not cool.

  • If the unit runs but blows warm or room-temperature air, the reversing valve may be stuck in heat mode, or refrigerant may be low.
  • Do not attempt to add refrigerant or repair the reversing valve yourself. Refrigerant work requires EPA certification.
  • Good: Technician diagnoses and repairs.
  • Bad: Continuing to run with low refrigerant can damage the compressor—call a pro.

When to get help

Call an HVAC technician if:

  • The thermostat is set correctly and the filter is clean but cooling is weak or absent.
  • The outdoor unit does not run and power checks pass.
  • You suspect refrigerant leak, reversing valve, or compressor failure.

Refrigerant work must be done by a licensed professional.

Verification

  • Cold air comes from the vents when the thermostat calls for cooling.
  • The outdoor unit runs and the fan spins.
  • Filter is clean and outdoor coil is free of debris.
  • No unusual noises or rapid cycling.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm cooling mode Check thermostat is set to Cool and below room temperature.
  2. Power and filter Check circuit breaker, disconnect, and air filter.
  3. Outdoor coil Clean debris from the outdoor coil.
  4. Reversing valve / refrigerant Technician tests reversing valve and refrigerant—no DIY.
  5. Call a pro Refrigerant, compressor, or reversing valve—licensed pro only.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Thermostat setting and mode
  • Whether outdoor unit runs
  • Air temperature at vents
  • Filter condition
  • Steps already tried

Is the thermostat set to Cool and below room temperature?

Heat pumps must be in cooling mode. Heat or Auto may run in heat mode.

Check the thermostat. Set to "Cool" or "AC." Set temperature below room temperature. Wait a few minutes. Yes: proceed to power and filter. No: set correctly and retest.

Yes No

You can change your answer later.

Set thermostat to Cool and retest

Set thermostat to Cool. Set temperature below room temperature. Wait 5 minutes. Check if cold air comes from vents. If yes, problem solved. If no, proceed to power and filter.

Done

Is power on and the filter clean?

Breaker, disconnect, and filter affect operation.

Check circuit breaker and disconnect switch. Replace dirty air filter. Good: power on, filter clean. Bad: fix and retest.

Yes No

You can change your answer later.

Fix power and filter

Reset breaker. Turn on disconnect. Replace filter. Retest. If cooling returns, done. If not, proceed to outdoor unit check.

Done

Does the outdoor unit run when cooling is requested?

The compressor and fan should run when the thermostat calls for cool.

When thermostat calls for cool, the outdoor unit should run. Yes: unit runs—check if air is cold. No: unit does not run—check contactor, capacitor, or call a pro.

Yes No

You can change your answer later.

Is cold air coming from the vents?

If unit runs but air is warm, reversing valve or refrigerant may be the issue.

Feel the air at the vents. Cold: cooling works—may have been thermostat or filter. Warm/room temp: reversing valve may be stuck, or refrigerant low. Technician must check—no DIY.

Yes No

You can change your answer later.

Cooling restored

Cooling is working. Check that the outdoor coil is clean for best performance.

Done

Call a technician

Call an HVAC technician for: outdoor unit not running (contactor, capacitor), weak or no cooling (reversing valve, refrigerant, compressor). Refrigerant work requires a licensed professional.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a heat pump run but not cool?
Common causes: thermostat set to heat instead of cool, dirty filter or outdoor coil, stuck reversing valve, low refrigerant, or failed compressor. Check thermostat and filter first, then have a technician test refrigerant and the reversing valve.
Can I fix a heat pump that will not cool myself?
You can check the thermostat setting, replace the filter, and clean the outdoor coil. Reversing valve and refrigerant work require a licensed HVAC technician. Do not attempt to add refrigerant yourself—it is regulated and requires certification.
When should I call a technician for a heat pump that will not cool?
Call if the thermostat is set correctly and the filter is clean but cooling is weak or absent. Call for refrigerant leaks, reversing valve issues, or compressor failure. Any refrigerant work must be done by a licensed professional.

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