Why Outside Heat Pump Unit Not Running (Troubleshooting Guide)

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Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

If your heat pump's outdoor unit isn't running, first check the thermostat settings, circuit breaker, and outdoor unit power switch. A tripped breaker or incorrect thermostat mode can prevent the unit from operating.

Confirm the symptom

Outdoor unit fan and compressor are completely silent
Indoor air handler is running but no heat/cooling
Thermostat shows no system activity
No clicking or humming sounds from outdoor unit
Unit worked yesterday but not today

Tools and prep

Voltage tester
Screwdriver set
Flashlight
Thermal imaging camera (optional)
Safety gloves and goggles
Turn off power at circuit breaker before inspecting
Clear debris around outdoor unit

Step-by-step troubleshooting

Check Thermostat Settings
Do: Verify thermostat is set to HEAT or COOL mode and temperature is outside current room temp
Observe: Thermostat shows correct mode and temperature
Means: Thermostat is functioning properly
Next: Check circuit breaker
Check Circuit Breaker
Do: Locate HVAC breaker and check if tripped
Observe: Breaker is in ON position
Means: Power is available to system
Next: Check outdoor unit power switch
Check Outdoor Unit Power Switch
Do: Look for a disconnect switch near outdoor unit and verify it's ON
Observe: Switch is in ON position
Means: Unit has power
Next: Test for power at unit
Test for Power at Unit
Do: Use voltage tester on terminal block or disconnect box
Observe: Voltage tester shows 240V power
Means: Unit has electrical supply
Next: Inspect for error codes
Inspect for Error Codes
Do: Check indoor unit control board or outdoor unit for blinking LED codes
Observe: Error code is displayed
Means: System has specific fault
Next: Consult manual for code meaning
Check Airflow and Debris
Do: Inspect outdoor unit for leaves, ice, or debris blocking fan
Observe: No obstructions in fan or coils
Means: Unit can operate freely
Next: Check refrigerant lines for frost

Common causes

Cause	Likelihood	Difficulty	Part often needed
Tripped circuit breaker Overload or short circuit interrupted power	Common	Easy	None
Thermostat wiring issue Loose or disconnected wires prevent signal to outdoor unit	Sometimes	Easy–Medium	Thermostat wire connectors
Outdoor unit power switch off Manual disconnect was turned off for maintenance or safety	Common	Easy	None
Failed outdoor fan motor Motor burned out or capacitor failed, preventing fan operation	Sometimes	Medium	Fan motor or capacitor
Defective contactor relay Stuck or burned contacts prevent power from reaching compressor	Sometimes	Medium	Contactor relay
Low refrigerant charge Leak caused system to shut down for safety	Rare	Hard	Refrigerant (requires pro)
Frozen evaporator coil Poor airflow or defrost issue caused ice buildup	Sometimes	Medium	Defrost timer or coil sensor

Parts matching

Common replacement parts for outdoor unit issues include:

Contactor relay (if clicking but not starting)
Outdoor fan motor and capacitor
Thermostat wire connectors
Defrost control board
High-pressure switch

Always match part voltage and model number to your unit's specifications.

When to call a professional

Unit has error code you can't resolve
You smell burning or hear loud bangs
Unit trips breaker repeatedly
Refrigerant leak suspected (frost, hissing sound)
System is over 10 years old with frequent issues
You're unsure about electrical testing safety

FAQ

Can a bad thermostat cause outdoor unit not to run?

Yes, if thermostat is faulty or wired incorrectly, it won't signal the outdoor unit. Test with a backup thermostat if possible.

Why would outdoor unit run but not cool?

This usually indicates refrigerant issues, airflow problems, or a failing compressor—not a complete power failure.

How to reset a heat pump outdoor unit?

Turn off power at circuit breaker for 30 seconds, then restore power. Only try this once if unit is completely dead.

Is it normal for outdoor unit to run constantly?

No—constant runtime without cycling suggests thermostat issues, oversized unit, or temperature sensor problems.