Why the Heat Pump Outside Won’t Turn On: Causes and Fixes

The heat pump not turning on outside is a common winter and summer problem that can leave a home without heating or cooling. This article explains the most likely causes, step-by-step troubleshooting, safety checks, repair options, and preventative maintenance to restore reliable operation. It focuses on practical, accurate guidance for U.S. homeowners and highlights when professional help is needed.

Symptom	Likely Cause	Quick Check
Outdoor Fan Not Running	Capacitor, Contactor, Fan Motor	Listen For Clicking, Inspect For Power
Unit Completely Dead	Breaker, Disconnect, Control Board	Check Breaker, Outdoor Disconnect
Clicks, Then Nothing	Failed Contactor, Capacitor, Compressor Issue	Watch For Repeated Clicking
Frozen Outdoor Coil	Defrost Failure, Low Refrigerant	Inspect Coil; Thaw If Iced
Runs Briefly Then Stops	Short Cycle, Safety Switch, Low Refrigerant	Check For Water, Ice, Refrigerant Smell

How A Heat Pump Outside Unit Works

A heat pump outside unit contains the compressor, outdoor coil, fan, contactor, capacitors, and other electrical controls that move heat between indoors and outdoors. In cooling mode it rejects heat to the outside; in heating mode it absorbs heat from outdoor air. The thermostat signals the indoor air handler, which in turn commands the outdoor unit to run.

Understanding these components helps isolate why the heat pump not turning on outside may occur. Power, control signals, and mechanical components must all function for the unit to operate.

Common Reasons The Heat Pump Outside Won’t Turn On

Power Supply Issues

The outdoor unit requires a dedicated 240V (or 208V) supply and a functioning circuit. Tripped breakers, a blown fuse in the outdoor disconnect, or an open safety disconnect will prevent the unit from powering up.

Check the home’s main electrical panel for tripped breakers and the outdoor disconnect switch, which is often located near the unit. Power loss is the most frequent reason the outdoor unit appears dead.

Thermostat Or Control Signal Problems

If the thermostat is not sending the correct call for heat or cool, the outdoor unit will stay off. Wiring issues, incorrect thermostat mode, or a failed control board in the indoor air handler can prevent the call from reaching the condenser.

Verify the thermostat is in the correct mode and set above or below room temperature to initiate a call. Confirm the indoor unit operates—if indoor blower runs but outdoor does not, the control signal may be interrupted.

Contactor Or Start Relay Failure

The contactor is an electromechanical switch that supplies high voltage to the compressor and fan when the unit is commanded to run. A burned or stuck contactor will stop power flow, leaving the heat pump not turning on outside.

Common indications include a brief click when the thermostat changes, or visible pitting on the contactor contacts. Replacing the contactor is often a straightforward repair for a licensed technician.

Capacitor Or Motor Problems

The start/run capacitor provides the extra torque motors need to start. A weak or failed capacitor prevents the outdoor fan or compressor from starting, sometimes resulting in humming or clicking sounds without rotation.

Symptoms include the fan motor not starting while receiving power, or the compressor attempting to start and failing. Capacitors are inexpensive but require safe replacement by someone comfortable working with high-voltage components.

Compressor Failure Or Seizure

The compressor is the heart of the heat pump. A seized or electrically failed compressor stops refrigerant flow and will keep the outdoor unit from starting. Attempting to restart a seized compressor can trip breakers repeatedly.

Compressor failures are expensive to repair and may lead to unit replacement depending on age and warranty status.

Low Refrigerant Or Refrigerant Leak

Low refrigerant levels can cause pressure safety switches to prevent the heat pump outside from turning on to avoid damage. In heating mode, low refrigerant often causes freezing on the outdoor coil and poor heating performance.

Only a certified HVAC technician should diagnose and repair refrigerant leaks and recharge the system due to EPA regulations and safety concerns.

Defrost Cycle Or Frozen Outdoor Coil

In heating mode, the outdoor coil may frost and the unit will periodically enter defrost. If the defrost control fails, the coil may remain frozen and the unit can stop running or perform poorly.

Thaw frozen coils safely and inspect for malfunctioning defrost controls or sensor issues. Never chip ice off the coil with tools that can damage fins.

Safety Switches And Float Switch Triggers

Condensate overflow switches, drain pan floats, and high-pressure or low-pressure safety switches can interrupt operation. A tripped float switch due to blocked drains will stop the system until cleared.

Check condensate drains and the area around the air handler for water. Reset or repair drainage issues before attempting to restart the system.

Step-By-Step Troubleshooting For A Heat Pump Not Turning On Outside

1. Confirm Thermostat Call: Ensure thermostat is in the correct mode and set to call for heat or cooling. Replace batteries if needed.
2. Check Indoor Unit: Verify indoor blower runs when the thermostat calls. If not, problem may be with thermostat wiring or air handler.
3. Inspect Breakers & Disconnect: Look for tripped breakers and open the outdoor disconnect to ensure the fuses are intact. Reset tripped breakers carefully.
4. Listen For Clicks: When the thermostat calls, listen for a click at the outdoor unit. No click suggests control power or contactor problems.
5. Look For Visible Damage: With power off, inspect wiring for burn marks, melted insulation, or loose connections.
6. Observe Fan & Compressor: If the fan doesn’t spin but the compressor hums, suspect a capacitor or start relay issue.
7. Check For Ice: If the outdoor coil is iced, thaw it and investigate defrost controls and refrigerant charge.
8. Note Error Codes: Some modern systems flash diagnostic codes on the indoor board or thermostat—record codes before resetting.
9. When In Doubt, Shut Off: If there is burning smell, smoke, or repeated tripping, turn off power and call a technician.

When To Call A Professional

Some issues are safe for homeowners to check, like thermostat settings, breakers, or visible ice, but many heat pump repairs involve hazardous voltages and refrigerants. Contact a licensed HVAC technician when:

• There Is No Power Despite Resetting Breakers
• Electrical Components Show Damage Or Burning
• Capacitors, Compressors, Or Contactors Need Replacement
• There Is A Suspected Refrigerant Leak Or Low Charge
• The System Is Under Warranty And Parts Need Replacement

Professional diagnosis will often include multimeter tests, pressure readings, and applying manufacturer-specific troubleshooting procedures.

Estimated Repair Costs And Replacement Considerations

Costs vary by region and system. Typical ranges in the U.S. include: contactor replacement $150–$400, capacitor $100–$300, fan motor $300–$800, compressor replacement $1,000–$3,000, full outdoor unit replacement $2,500–$8,000 depending on capacity and efficiency.

When repair cost exceeds 50% of replacement cost, or the system is older than 10–15 years, replacement may be more economical. Energy efficiency and refrigerant type also influence replacement decisions.

Maintenance To Prevent The Heat Pump Outside From Not Turning On

• Schedule Annual Professional Maintenance: Cleaning, lubrication, and inspection reduce failures.
• Change Indoor Filters Regularly: Clogged filters increase system strain and can lead to icing and shutdowns.
• Keep Outdoor Unit Clear: Remove debris, leaves, and snow around the unit to ensure airflow.
• Inspect Electrical Connections: Tighten loose terminals during maintenance visits to prevent arcing.
• Check Drainage: Clear condensate drains and check for standing water near the air handler.

Safety Tips For Homeowners

Always shut power to the system before inspecting electrical components. Do not attempt capacitor or compressor repairs without training; these components can store dangerous voltage after power is removed.

If the system repeatedly trips breakers or emits burning smells, leave it off and call a technician. Prioritize safety over saving time when diagnosing complex electrical or refrigerant-related failures.

How Warranties And HVAC Service Plans Help

Many new heat pumps carry manufacturer warranties on compressors and parts for 5–10 years, and extended warranties or service plans can cover labor. Regular maintenance often is required to keep warranties valid.

Service plans provide annual tune-ups that catch issues early and can reduce the chance the heat pump not turning on outside unexpectedly.

Checklist For Technicians To Diagnose A Heat Pump Not Turning On Outside

Step	What To Check
Power Verification	Confirm voltage at outdoor disconnect and contactor with multimeter
Control Signal	Verify 24VAC call from thermostat to contactor coil
Contactor Condition	Inspect contacts for pitting; measure continuity
Capacitor Test	Measure capacitance and ESR; replace if out of tolerance
Motor & Compressor	Check amp draw, locked rotor, and winding resistance
Refrigerant Pressures	Analyze pressures for proper charge and leaks
Defrost & Sensors	Evaluate defrost board, sensors, and timers

Key Takeaways For Homeowners

If the heat pump outside won’t turn on, start with basic checks: thermostat settings, breakers, and the outdoor disconnect. Major causes include power issues, failed contactors or capacitors, compressor problems, defrost failures, and low refrigerant.

Simple checks can identify obvious problems, but many repairs require licensed technicians due to electrical hazards and refrigerant regulations. Regular maintenance and timely repairs minimize the chances the heat pump not turning on outside will occur at inconvenient times.

For persistent or complex failures, documentation of symptoms, sounds, and any error codes speeds diagnosis and reduces repair time when the technician arrives.