Heat Pump Compressor Not Turning On: Troubleshooting and Fixes

The heat pump compressor not turning on can leave a home without heating or cooling and may signal electrical, mechanical, or control failures. This article guides homeowners through safe, logical troubleshooting steps, common causes, diagnostic tips, and when to call a licensed technician.

Symptom	Likely Cause	DIY Check
Outdoor Unit Silent	Tripped Breaker, Blown Fuse	Check circuit breaker and disconnect
Fan Runs But Compressor Idle	Capacitor, Contactor, Compressor Failure	Listen for humming, inspect capacitor
System Short Cycles	Low Refrigerant, Control Board	Check refrigerant lines for frost, call tech

Why The Heat Pump Compressor Not Turning On Matters

The compressor is the heart of a heat pump, circulating refrigerant to transfer heat. When the heat pump compressor not turning on, the system cannot move heat efficiently, causing temperature loss, reduced comfort, and higher energy usage.

Immediate impacts include lack of heating or cooling, potential frozen coils, and possible further damage if the issue persists.

Safety First: Precautions Before Troubleshooting

Working on HVAC systems involves electrical and refrigerant hazards. Always disconnect power at the outdoor unit shutoff and the main breaker before accessing components.

Only perform visual inspections and non-invasive checks unless qualified. Do not attempt refrigerant work, compressor replacement, or live-voltage testing without proper certification and tools.

Initial Quick Checks

Start with simple checks that resolve many issues quickly. These steps help determine whether the problem is a minor interruption or a significant failure.

Verify Thermostat Settings

Ensure the thermostat is set to heat or cool as appropriate and the temperature setpoint is above or below indoor temperature. Confirm the thermostat is on battery power or receiving 24V power.

Check Circuit Breakers And Fuses

Inspect the main breaker and the dedicated heat pump breaker in the electrical panel for tripped switches. Reset any tripped breaker and test the system.

Check the outdoor disconnect fuse at the unit; replace a blown fuse with the correct type and rating.

Power At The Outdoor Unit

Confirm the outdoor unit has power. If the outdoor fan or lights are off, the unit may lack power. Use a non-contact voltage tester or call a professional if uncertain.

Common Causes And How To Diagnose Them

This section covers the most frequent reasons for a heat pump compressor not turning on and practical diagnostic tips to identify each cause.

Tripped Or Faulty Breakers And Fuses

Electrical overloads, shorts, or aged breakers can trip repeatedly. Replace worn breakers and investigate causes of recurrent tripping, such as motor or compressor faults.

Bad Start Or Run Capacitor

The start capacitor provides the extra torque the compressor needs to start. A failing capacitor often results in a humming sound or the compressor not engaging.

Visually inspect the capacitor for bulging, leaking, or corrosion. A multimeter with capacitance measurement confirms failure; replace with manufacturer-specified ratings.

Contactor Problems

The contactor is an electromagnetic switch that sends power to the compressor. Pitted or welded contacts can prevent the compressor from receiving power.

Visual signs include burned or pitted contacts. Replace the contactor if contacts are damaged or if the coil fails to pull in when commanded.

Control Board Or Relay Failures

The outdoor unit’s control board or relays can fail, preventing the compressor from receiving control signals. Symptoms include no response at all when the thermostat calls for the compressor.

Diagnostic steps include checking for 24V control signal from the thermostat and inspecting the control board for burnt components or error LEDs. Replace boards based on manufacturer diagnostics.

Thermostat Or Low-Voltage Wiring Issues

Loose, corroded, or disconnected low-voltage wires between the thermostat and outdoor unit can stop compressor activation.

Inspect wiring at the thermostat and terminal block for secure connections and 24V presence. Replace damaged wires and ensure proper terminal labeling (R, C, Y, O/B as applicable).

Refrigerant Problems And Pressure Switches

Low refrigerant or abnormal pressures can trigger safety switches that block compressor operation. A frozen evaporator coil or visible oil stains can indicate leaks.

Only certified technicians should measure refrigerant pressures and recharge systems. If low refrigerant is confirmed, expect leak detection, repair, and a proper recharge.

Compressor Electrical Failures

Internal motor windings, shorted components, or seized compressors can prevent startup. Symptoms include humming, clicking, or a persistent lack of power despite correct control signals.

Technicians may perform insulation resistance (megger) and locked-rotor amperage tests. A failed compressor often requires replacement, which is an expensive repair with refrigerant reclamation requirements.

Step-By-Step Diagnostic Workflow

Follow a logical sequence to isolate the problem efficiently. This workflow reduces unnecessary parts replacement and identifies when professional service is needed.

1. Confirm thermostat call and correct settings.
2. Check breakers, outdoor disconnect and fuses.
3. Listen for outdoor fan operation; note any humming or clicking.
4. Inspect capacitor and contactor visually for damage.
5. Verify 24V control signal at outdoor unit when thermostat calls.
6. If electrical control is present but compressor won’t start, suspect capacitor, contactor, or compressor motor.
7. If system has abnormal pressures or frozen coils, stop and call an HVAC tech for refrigerant diagnostics.

When The Outdoor Fan Runs But Compressor Won’t Start

This common scenario narrows the fault to components specifically related to compressor starting. The outdoor fan can run on a separate circuit or share the start capacitor.

Likely causes include a failed start capacitor, a faulty contactor that doesn’t connect the compressor circuit, or the compressor itself being electrically damaged.

Testing And Replacing The Capacitor And Contactor

Capacitor testing uses a capacitance meter after discharging the device safely. Improper handling can shock or damage components.

Replace capacitors with unit-matched microfarad and voltage ratings. Replace contactors with compatible amperage and coil voltage ratings and ensure proper wiring and tight connections.

Control Board, Relays, And Thermostat Fixes

Control board failures can present as no compressor control or intermittent faults. Check for diagnostic LEDs and error codes on the board.

Thermostat problems are often resolved by replacing batteries, recalibrating, or swapping for a known-good thermostat. Smart thermostats may need configuration to support heat pump reversing valves and stage control.

Costs And Timeframes For Repairs

Repair	Typical Cost Range (US)	Typical Time
Capacitor Replacement	$75 – $250	30–90 Minutes
Contactor Replacement	$100 – $350	30–90 Minutes
Control Board Replacement	$200 – $800	1–3 Hours
Compressor Replacement	$1,200 – $4,500+	4–10+ Hours
Refrigerant Leak Repair + Recharge	$200 – $1,500+	1–6+ Hours

Costs vary by region, unit type, warranty coverage, and refrigerant type. Compressor replacement typically includes refrigerant recovery and system evacuation, increasing labor and compliance requirements.

Preventive Maintenance To Avoid Future Failures

Regular maintenance extends equipment life and reduces unexpected failures. Recommended actions include biannual inspections, cleaning coils, checking electrical connections, and testing capacitors and contactors.

Change air filters regularly, maintain airflow by keeping vents clear, and keep outdoor coils free of debris to reduce compressor strain.

When To Call A Licensed Technician

Call a professional when electrical diagnostics beyond breaker checks are needed, refrigerant handling is required, or the compressor shows signs of internal failure.

Immediate professional service is recommended if there is burning smell, smoke, repeated breaker trips, or the compressor hums but won’t start after simple checks.

Warranty, Replacements, And Environmental Considerations

Check the unit’s warranty before major repairs; some components may be covered. Compressors and refrigerant work often have manufacturer or extended warranty implications.

Refrigerant recovery is federally regulated in the U.S.; only certified technicians should handle refrigerants to comply with EPA rules and avoid environmental harm.

Frequently Asked Questions

Why Does The Compressor Hum But Not Start?

Humming usually indicates the compressor motor is receiving power but lacks starting torque, often due to a failed start capacitor or internal mechanical seizure.

Can A Homeowner Replace A Capacitor?

Capacitor replacement is borderline for experienced DIYers with electrical knowledge. Proper discharge, measurement, and installation are critical. If unsure, hire a technician.

Is It Worth Repairing An Old Compressor?

Consider age, efficiency, and warranty. If the unit is more than 10–15 years old, replacing the entire heat pump may be more cost-effective and energy-efficient than replacing a compressor.

How Long Will Repairs Take?

Minor repairs like capacitor or contactor replacement often finish within a couple hours. Major tasks like compressor replacement or leak locating can take multiple hours or a day, depending on parts availability.

Resources And Next Steps

Homeowners should keep a maintenance log, know the unit model and serial number, and gather warranty documents before contacting service. Request written estimates and ensure technicians are licensed and EPA certified for refrigerant work.

For recurring or complex issues, seek a second opinion or consult a certified HVAC contractor for diagnostics, diagnostics reports, and recommended options.

Key takeaway: A heat pump compressor not turning on often has diagnosable causes—start with power and thermostat checks, inspect capacitors and contactors, and call a professional for refrigerant or major electrical faults.