Direct answer: When a heat pump outdoor unit is not running, the most common homeowner-level causes are a thermostat call issue, a tripped breaker or pulled disconnect, a clogged filter causing a freeze-up, or a safety shutdown after the system struggled. If the indoor blower runs but the outdoor section stays dead, check power and airflow first before assuming a bad major part.
Most likely: Start with the thermostat mode and setpoint, the air filter, the indoor and outdoor breakers, and the outdoor disconnect. After that, look for ice on the outdoor coil or refrigerant lines, or a humming outdoor unit that will not start.
A dead outdoor unit can look worse than it is. Sometimes it is just a lost power feed or a system that iced up and locked itself out. Reality check: if the outdoor unit is completely silent, power is usually the first thing to prove. Common wrong move: changing parts before checking the disconnect, breaker, and filter.
Don’t start with: Do not start by opening electrical panels, pushing contactors by hand, or replacing a capacitor or contactor just because the fan is not spinning.
Air comes from the vents, but the outdoor fan and compressor never start. You may hear the thermostat click inside, but nothing happens outside.
Start here: Start with thermostat settings, breaker status, and the outdoor disconnect because this pattern often points to a lost call or lost power.
You hear a hum, buzz, or repeated clicking at the outdoor unit, but the fan does not spin up and the system does not heat or cool normally.
Start here: Start with a full power-off reset and a visual check for a swollen capacitor area or debris-bound fan, then stop before opening electrical compartments.
The system worked for a while, then the outdoor section shut down while the indoor blower kept moving air or the thermostat kept calling.
Start here: Start by checking for a dirty filter, blocked coil, or ice on the outdoor unit or refrigerant lines because freeze-up and overload shutdowns are common here.
The thermostat appears on, but neither the indoor blower nor the outdoor unit starts when you call for heating or cooling.
Start here: Start at the thermostat, furnace or air-handler switch, and breakers because the problem may be upstream of the outdoor unit.
A tripped breaker, blown disconnect fuse, loose disconnect, or service switch issue can leave the outdoor unit completely dead while the thermostat still appears normal.
Quick check: Confirm the indoor and outdoor breakers are fully on, and make sure the outdoor disconnect block is seated correctly.
If the thermostat is in the wrong mode, the setpoint is too close to room temperature, or the indoor equipment is not passing the call through, the outdoor unit never gets told to start.
Quick check: Set the thermostat several degrees past room temperature in the correct mode and listen for the indoor unit to respond.
A badly clogged filter or blocked coil can ice the system or overwork it until the outdoor section stops. This is especially common if airflow has been weak lately.
Quick check: Pull the filter and inspect it in good light. Look for ice on the outdoor coil, suction line, or indoor refrigerant line insulation.
A failed heat pump run capacitor, stuck heat pump contactor, seized heat pump condenser fan motor, or compressor problem can cause humming, clicking, or a unit that tries but will not run.
Quick check: With power left on only long enough to observe safely, listen for humming or repeated clicking at the outdoor unit, then shut it back off and call for service if that is what you hear.
A lot of no-run calls come down to a thermostat setting, a recent battery issue, or a mode mismatch. This is the safest place to start.
Next move: If the outdoor unit starts after the delay, the issue was likely a control setting or temporary lockout. If the indoor blower runs but the outdoor unit stays off, move to power checks. If nothing runs anywhere, focus on indoor power or thermostat issues first.
What to conclude: This separates a true outdoor-unit failure from a thermostat or whole-system call problem.
A heat pump outdoor unit can lose power at the breaker or disconnect and look completely dead. This is common after storms, yard work, or recent service.
Next move: If the outdoor unit starts and runs normally, you likely had a simple power interruption. If the breaker trips again, the disconnect is hot or damaged, or the outdoor unit is still dead, stop there and treat it as an electrical fault.
What to conclude: A one-time reset can solve a nuisance trip, but a repeat trip usually points to a shorted component, locked motor, or compressor issue that needs testing.
A heat pump that has iced up or overheated from poor airflow may shut the outdoor section down or run so poorly it seems dead. This is one of the few useful homeowner checks that can prevent a wasted service call.
Next move: If the unit runs normally after a clean filter and full thaw, the immediate cause was likely airflow restriction. If ice returns quickly or the outdoor unit still will not start, the problem is beyond basic maintenance and may involve refrigerant, a sensor, or an electrical fault.
Once power and airflow are ruled out, the next useful clue is whether the outdoor unit is trying to start. Humming, clicking, or a fan blade that never gets moving often points to an outdoor electrical or motor problem.
Next move: If the unit starts cleanly and keeps running after a reset and thaw, monitor it through a full cycle. If it hums, clicks repeatedly, or trips power, schedule service and describe exactly what you heard and saw.
After thermostat, power, filter, and ice checks, there is not much safe DIY left on a heat pump outdoor unit. A clean restart can confirm whether the problem was temporary or whether you are dealing with a real component failure.
A good result: If it starts and completes a normal cycle, keep using it but watch for repeat shutdowns over the next day or two.
If not: If the outdoor unit still will not run, the practical next step is professional service. Give the tech the pattern you observed so they can go straight to electrical testing or refrigerant diagnosis.
What to conclude: At this point you have ruled out the common homeowner fixes and narrowed the fault enough to avoid random part replacement.
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That usually points to one of four things: the outdoor unit lost power, the thermostat call is not reaching it, the system iced up and shut down, or an outdoor electrical component failed. Start with thermostat settings, breaker and disconnect checks, then look for a dirty filter or ice.
Yes. A badly clogged heat pump air filter can choke airflow enough to freeze the system or trigger a shutdown. It will not cause every no-run problem, but it is common enough that it should be checked early.
Yes, once. Turn it fully off and back on. If it trips again, stop. A repeat trip usually means a real electrical or mechanical fault, and continuing to reset it can make the damage worse.
It can be, but do not treat that as a sure thing. A failed heat pump run capacitor is common, but a stuck contactor, failing heat pump condenser fan motor, or compressor problem can look similar. On a heat pump, that is usually a service call rather than a blind DIY parts swap.
Not in cooling mode. Shut it off and let it thaw, then correct any obvious airflow problem like a dirty filter. If the ice comes back quickly, the system needs service because the cause is often beyond basic cleaning.
Then the problem may not be the outdoor unit at all. Check thermostat power, indoor unit power, service switches, and breakers first. If the thermostat is blank or the indoor equipment is dead, start there.