Nothing at the outdoor unit runs
No fan, no compressor sound, and no vibration at the outdoor unit when the thermostat is calling.
Start here: Start with thermostat mode and setpoint, then check the HVAC breakers and the outdoor disconnect.
Heat Pump Troubleshooting
Heat Pump Compressor Not Starting
Direct answer: If the heat pump compressor is not starting, the most common homeowner-level causes are no real call from the thermostat, a tripped breaker or disconnect, a badly restricted filter or coil causing a lockout, or an outdoor unit sitting in a defrost or fault condition. If the outdoor fan runs but the compressor stays silent or just hums, the problem is usually beyond safe DIY.
Most likely: Start by confirming the thermostat is actually calling, the indoor filter is not packed, and the outdoor unit has full power. Then separate three lookalikes: nothing outside runs, only the outdoor fan runs, or the unit is iced over.
A compressor that will not start can look like a dead heat pump, weak heat, or a system that blows room-temperature air. Reality check: many compressor complaints turn out to be a power, airflow, or control issue you can spot from the ground. Common wrong move: resetting breakers over and over when the outdoor unit is trying to tell you there is a real electrical or mechanical fault.
Don’t start with: Do not start by replacing the compressor, capacitor, contactor, or control parts. On a heat pump, those are high-risk guesses and often not homeowner-safe repairs.
If nothing outside runsCheck thermostat demand, breaker position, and the outdoor disconnect before anything else.
If the outdoor fan runs but no heat change happensStop at basic checks and plan for service, because that points toward a compressor-start or sealed-system problem.
Last reviewed: 2026-04-05
What this usually looks like
Outdoor fan runs but compressor does not
You hear the fan outside, but there is no deeper compressor sound and the air indoors does not get properly warm or cool.
Start here: Check for icing, dirty filter, and obvious fault behavior, then stop before opening electrical compartments.
Compressor tries, hums, or clicks off
The outdoor unit makes a short hum, click, or brief start attempt, then quits.
Start here: Shut the system off and do not keep forcing restarts. That pattern needs a technician.
System heats only with auxiliary heat
The house warms slowly, AUX or EM heat shows often, and the outdoor unit may run oddly or not at all.
Start here: Confirm the outdoor unit is powered and not iced over, then decide whether this is a compressor issue or an auxiliary-heat issue.
Most likely causes
1. Thermostat is not sending a real heating or cooling call
A schedule, wrong mode, dead batteries, or a small setpoint difference can leave the outdoor unit idle even though the system seems 'on.'
Quick check: Set the thermostat to the needed mode and move the setpoint at least 3 to 5 degrees past room temperature.
2. Power is lost to the outdoor heat pump unit
A tripped breaker, pulled disconnect, or partial power loss can leave the compressor off or the whole outdoor unit dead.
Quick check: Check for a tripped HVAC breaker and make sure the outdoor disconnect is fully seated and not switched off.
3. Airflow restriction or icing has pushed the system into a protective shutdown
A packed filter, blocked coil, or heavy frost can make a heat pump stop normal compressor operation to protect itself.
Quick check: Inspect the indoor filter, return grilles, and the outdoor coil for dirt, snow, leaves, or solid ice.
4. Outdoor electrical or compressor-start failure
If the fan runs but the compressor does not, or it hums and trips out, the trouble is often in the start circuit, controls, or the compressor itself.
Quick check: Listen for humming, clicking, or repeated short attempts, then stop at observation only.
Step-by-step fix
Step 1: Make sure the thermostat is actually calling for the heat pump
A lot of no-compressor calls start with the thermostat not asking the outdoor unit to run long enough or in the right mode.
- Set the thermostat to HEAT if you need heat, or COOL if you are checking cooling operation.
- Raise or lower the setpoint 3 to 5 degrees past room temperature so the system has a clear demand.
- If the thermostat uses batteries, replace them if the display is dim, blank, or acting erratic.
- Wait several minutes after changing settings. Some systems have a built-in delay before the outdoor unit starts.
- If EMERGENCY HEAT is on, turn it off unless you are intentionally bypassing the heat pump.
Next move: If the outdoor unit starts after a short delay and normal heating or cooling returns, the issue was likely settings, timing, or thermostat demand. If there is still no outdoor response, move on to power checks before assuming a bad compressor.
What to conclude: This separates a control-call problem from a true outdoor unit failure.
Stop if:
- The thermostat display is blank and does not recover after fresh batteries or power checks.
- You smell burning plastic or hear buzzing from the thermostat or air handler.
Step 2: Check for lost power to the outdoor unit
A heat pump compressor cannot start without full outdoor power, and this is one of the most common safe checks a homeowner can make.
- Check the main HVAC breakers in the electrical panel. A tripped breaker usually sits between ON and OFF.
- If a breaker is tripped, reset it once by switching it fully OFF and then back ON.
- Look at the outdoor disconnect near the unit and make sure it is inserted or switched fully on.
- Listen again at the outdoor unit after restoring power: note whether nothing runs, only the fan runs, or the unit tries to start and quits.
Next move: If the unit starts and runs normally after power is restored, monitor it through a full cycle. If the breaker trips again, or the unit still will not start, stop resetting and continue with non-invasive checks only.
What to conclude: Repeated trips or partial operation point to an electrical fault, seized load, or another problem that is not fixed by more resets.
Stop if:
- The breaker trips immediately or within minutes of restart.
- The disconnect looks burned, loose, wet inside, or damaged.
- You are not comfortable working around the electrical panel or outdoor disconnect.
Step 3: Rule out airflow restriction, dirt, and icing
A heat pump that is starved for airflow or buried in frost can shut down compressor operation or perform so poorly that it looks like the compressor never came on.
- Turn the system off at the thermostat before inspecting the filter and outdoor coil.
- Pull the indoor air filter and replace it if it is packed with dust or matted over.
- Make sure return and supply grilles are open and not blocked by rugs, furniture, or heavy dust buildup.
- Check the outdoor unit for leaves, lint, snow, or grass packed against the coil. Clear debris gently by hand.
- If the outdoor coil is dirty, rinse the exterior fins lightly with plain water from the inside-safe direction you can access without opening panels. Do not bend fins or soak electrical compartments.
- If the unit is heavily iced over, shut the system off and let it thaw naturally. Do not chip ice off the coil or tubing.
Next move: If the unit restarts after thawing or after restoring airflow, and it now runs through a normal cycle, the shutdown was likely protective rather than a failed compressor. If the outdoor fan runs but the compressor still does not start, or icing returns quickly, the problem has moved past basic maintenance.
Stop if:
- The coil is encased in solid ice or the refrigerant lines are heavily frozen.
- You see oily residue on refrigerant tubing or around the outdoor unit.
- The unit restarts but makes loud humming, clanking, or hard-start noises.
Step 4: Separate fan-only operation from a true dead outdoor unit
This is the key split. A completely dead outdoor unit often points to power or control loss. A running fan with no compressor points much more strongly to a pro-level fault.
- Stand near the outdoor unit after a thermostat call and listen carefully from a safe distance.
- If you hear only the fan blade and no deeper compressor sound or cabinet vibration, note that as fan-only operation.
- If you hear a brief hum, click, or repeated start attempts, shut the system off at the thermostat.
- If nothing at all happens outside but the indoor blower runs, note that as no outdoor operation.
- Check whether AUX heat is carrying the house. That can hide the fact that the heat pump section is not working.
Next move: If your observations clearly match one pattern, you can avoid random part swapping and make the next move cleaner. If the sounds and behavior are inconsistent, leave the system off and have it diagnosed under load by a technician.
Stop if:
- The unit hums loudly and then clicks off.
- The breaker has tripped more than once.
- You notice a hot electrical smell, smoke, or scorched wiring insulation.
Step 5: Shut it down and call for service when the compressor branch is clear
Once the safe checks are done, the remaining likely causes involve live electrical testing, refrigerant diagnosis, or compressor condition. Those are not good guess-and-buy jobs.
- Turn the thermostat off to stop repeated start attempts if the compressor hums, clicks, or the breaker is unstable.
- Tell the technician exactly what you observed: nothing runs outside, fan-only operation, icing, humming, or repeated breaker trips.
- If the house still needs heat, use auxiliary or emergency heat only if your system supports it and it runs safely.
- If your issue is really weak heat with normal outdoor operation, continue with /heat-pump-air-from-vents-not-warm-enough.html instead.
- If your issue is mostly poor airflow at the registers, continue with /heat-pump-airflow-weak-in-heat-mode.html instead.
A good result: A clean symptom report usually shortens diagnosis and helps avoid unnecessary parts.
If not: If the home cannot maintain safe indoor temperature, arrange prompt HVAC service rather than forcing more restarts.
What to conclude: At this point the likely fixes are in the outdoor electrical or refrigerant side, and that is where safe DIY ends on most heat pumps.
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FAQ
Why is my heat pump fan running but the compressor is not?
That usually points to a problem beyond basic DIY, such as a compressor-start issue, an outdoor control fault, or a sealed-system problem. First rule out a dirty filter, heavy icing, and obvious power issues. If the fan keeps running without the compressor, stop there and schedule service.
Can a dirty filter keep a heat pump compressor from starting?
Yes, indirectly. A badly restricted heat pump air filter can cause poor airflow, icing, or protective shutdown behavior that makes the compressor stop or perform erratically. It is one of the first things worth checking because it is safe, common, and cheap to fix.
Should I keep resetting the breaker if the compressor will not start?
No. Reset it once only. If it trips again, the system likely has a real electrical or mechanical fault. Repeated resets can overheat wiring and make the repair more expensive.
Does ice on the outdoor unit mean the compressor is bad?
Not by itself. Light frost can be normal in heat mode, but heavy ice buildup is not. A dirty filter, blocked airflow, defrost problem, or refrigerant issue can all cause icing. If the unit is heavily iced and the compressor still will not start, shut it down and have it checked.
Can I replace a heat pump capacitor or contactor myself to get the compressor going?
On a heat pump, that is usually not a good homeowner repair path unless you are trained and have already confirmed the fault safely. Those parts live in a high-voltage compartment, and guessing wrong is common. This page is better used to confirm when the problem has moved into pro service.