Heat Pump Outside Unit Not Running: Troubleshooting and Common Fixes

A heat pump outside unit not running can leave a home without heating or cooling and point to causes ranging from simple power issues to compressor failures. This article guides readers through step-by-step troubleshooting, likely causes, DIY checks, professional diagnostics, maintenance tips, and cost ranges for common repairs to help restore outdoor unit operation safely and efficiently.

Symptom	Likely Cause	Quick Action
Outdoor fan won’t run	Capacitor, motor, contactor	Check power, listen for hum, inspect capacitor
Unit completely dead	Tripped breaker, switch off, no power	Reset breaker, check disconnect, call electrician if needed
Clicks but won’t start	Faulty contactor, capacitor, compressor	Test capacitor/contactor, call HVAC pro
Runs briefly then stops	Overload, low refrigerant, defrost control	Check error codes, monitor run cycle

How Heat Pump Outside Units Operate

A heat pump outside unit contains the compressor, outdoor fan, reversing valve, and refrigerant lines that move heat between indoors and outdoors. The indoor thermostat signals a call for heating or cooling, the outdoor unit receives 240V power, and the contactor closes to energize the compressor and fan. Understanding this flow helps narrow down where failure occurs.

Initial Safety And Preparation Steps

Before any inspection, turn off power to the outdoor unit at the service disconnect and the main breaker. Wear protective gloves and safety glasses when accessing electrical components. If the homeowner is unfamiliar with electrical work, contacting a qualified HVAC technician or electrician is recommended.

Quick Checks To Perform First

Check Thermostat Settings

Confirm the thermostat is set to the correct mode (Heat, Cool, or Auto) and temperature. Replace batteries if the display is blank and verify the thermostat is truly sending a call for the heat pump.

Inspect Power Supply

Verify the outdoor unit has power by checking the home’s main breaker panel and the dedicated 240V breaker for the HVAC system. Look for a tripped breaker or blown fuse and reset it only once. Also inspect the outdoor disconnect switch near the unit.

Listen For Sounds

When calling for operation, listen for a click at the outdoor unit. A click often indicates the contactor is trying to engage. Silence may point to no power, while a hum without rotation often indicates capacitor or motor failure.

Common Causes And How To Diagnose Them

1. Power Issues And Controls

Power problems are the most common reason an outdoor unit won’t run. A tripped breaker, blown fuse, or a turned-off disconnect switch will make the unit appear dead. After confirming power at the breaker, test for voltage at the contactor terminals if comfortable with multimeters.

2. Thermostat Or Low-Voltage Wiring Faults

Faulty thermostat wiring or a bad thermostat can prevent the outdoor unit from receiving the 24V control signal. Check the R and C terminals and the wire connections at the indoor air handler. Look for burnt or corroded low-voltage terminals.

3. Failed Contactor

The contactor is an electromechanical switch that lets line voltage reach the compressor and fan. If the contactor coil receives 24V but the unit doesn’t power up, the contactor contacts may be burnt or stuck. Inspect visually for pitting or melting.

4. Bad Start/Run Capacitor

Start and run capacitors provide the electrical boost that helps the compressor and fan motor start. Symptoms include a humming unit that won’t spin, intermittent starts, or clicking relays. Capacitors can be tested with a meter or replaced as a common DIY fix but must be discharged safely.

5. Outdoor Fan Motor Failure

If the compressor runs but the fan doesn’t, the fan motor or its capacitor is often the culprit. A seized bearing or burnt motor winding will prevent fan rotation and can cause overheating and short cycling.

6. Compressor Issues

A dead or locked compressor is serious and can occur from electrical failure, low refrigerant causing overheating, or mechanical faults. If the compressor doesn’t run and the contactor and capacitors are good, a professional diagnosis is required, including amp draw and pressure checks.

7. Refrigerant Problems

Low refrigerant due to leaks can cause the system to short cycle, fail to start, or run inefficiently. Refrigerant diagnosis requires specialized gauges, leak detection, and EPA-certified technicians to repair and recharge the system.

8. Defrost Or Control Board Malfunctions

Faulty defrost boards, safety switches, or the unit control board can prevent operation. Error codes on the air handler or outdoor board may indicate specific sensor failures or lockouts that require professional programming or replacement.

Step-By-Step Troubleshooting Flow

1. Confirm Thermostat Call: Ensure thermostat is signaling heat/cool.
2. Check Breakers/Disconnect: Reset tripped breakers and verify disconnect is on.
3. Listen For Clicks/Hums: Note sounds when call is made.
4. Inspect Contactor: Open unit, visually check contactor and voltage to coil (24V).
5. Test Capacitor: Look for bulging or leaking; meter-test or replace.
6. Check Fan Motor: Manually spin fan with power off for binding; test motor windings with meter.
7. Measure Compressor Amps: Compare to nameplate values; high amps suggest mechanical problems.
8. Look For Refrigerant Signs: Icing, hissing, or oil stains indicate leaks.
9. Read Error Codes: Consult HVAC control board manual for fault interpretation.

When To Call A Professional

A professional should be called when the issue involves high-voltage testing, compressor diagnosis, refrigerant handling, or complex electrical/control board faults. Do not attempt refrigerant repairs unless certified under EPA Section 608. If tests show a locked compressor, high current draw, or internal motor damage, a qualified technician should proceed.

Maintenance Tips To Prevent Outdoor Unit Failure

• Keep the outdoor unit clear of debris and vegetation to ensure airflow.
• Change indoor air filters regularly to reduce system strain.
• Schedule annual HVAC tune-ups before heating or cooling seasons.
• Check and tighten electrical connections during maintenance visits.
• Clear ice and snow in winter and ensure proper defrost function.

Estimated Repair Costs And Timeframes

Costs vary by region and model, but typical ranges are: capacitor replacement $100–$300, contactor $100–$250, fan motor $200–$600, refrigerant leak repair $200–$1,500 depending on leak location and amount, and compressor replacement $1,200–$3,500 or more. Diagnostics often take 1–3 hours and more extensive repairs can require 1–3 days.

Energy And Performance Considerations

A malfunctioning outdoor unit will reduce system efficiency and increase energy costs. Burnt contactors, failing capacitors, or low refrigerant elevate electrical consumption. Regular maintenance can preserve SEER-rated performance and extend equipment lifespan.

Common Error Codes And What They Mean

Error Code	Possible Meaning
U1/U2	Communication fault between indoor and outdoor units
CF	Compressor failure/locked rotor
OF	Outdoor fan motor fault
LF	Low refrigerant or low-pressure fault

Refer to the system’s service manual for manufacturer-specific codes and suggested remedies.

DIY Safety Checklist

• Turn off all power at the breaker and disconnect before opening the unit.
• Use insulated tools and a non-contact voltage tester when checking power.
• Do not touch capacitor terminals without discharging them properly.
• If unsure about electrical testing or refrigerant, stop and call a technician.

How To Choose A Qualified HVAC Technician

Select a technician who is licensed, insured, and EPA-certified for refrigerant handling. Request references, read reviews, and ask for written estimates. A reputable contractor will provide a clear diagnosis, repair options, and energy-saving recommendations.

Preventive Upgrade Options

Upgrading to a modern, high-efficiency heat pump can resolve recurring reliability problems and reduce utility bills. Consider variable-speed compressors, smart thermostats, and enhanced defrost controls for improved performance in cold climates.

Frequently Asked Questions

Why Does The Unit Click But Not Start?

Clicking often indicates the contactor receives a 24V signal but the compressor or fan fails due to a bad capacitor, stuck contactor, or internal motor failure.

Can A Homeowner Replace A Capacitor?

Capacitor replacement is a common homeowner repair but it involves stored energy and proper discharge. If the homeowner is confident with safety procedures and has the right tools, it can be done; otherwise hire a pro.

Is It Normal For The Unit To Short Cycle?

Short cycling may indicate overheating, low refrigerant, electrical issues, or incorrect thermostat settings. Continued short cycling can damage the compressor and should be diagnosed promptly.

Additional Resources And References

Manufacturers’ service manuals, the Air Conditioning Contractors of America (ACCA) guidelines, and EPA Section 608 resources provide authoritative technical and safety information. Homeowners should keep model and serial numbers available for technicians and warranty inquiries.

For persistent or complex faults—especially those involving the compressor, refrigerant, or high-voltage electrical systems—contact a licensed HVAC professional for safe, code-compliant repairs.