Common Heat Pump Error Codes and What They Mean
Heat pumps are more complex than standard air conditioners because they operate in both heating and cooling modes, using a reversing valve to change refrigerant flow direction. This additional complexity means they have additional fault conditions and error codes that air conditioners don't. Whether you own a Carrier, Trane, Lennox, Rheem, or another brand of heat pump, many of the core fault codes relate to the same systems — refrigerant circuit, defrost, reversing valve, and communication. This guide covers the most common heat pump error codes across major brands and what they mean.
Heat Pump Fault Systems: What Can Go Wrong
Heat pumps can experience most of the same faults as air conditioners (high/low pressure, compressor issues, fan faults, communication errors) plus several heat-pump-specific issues:
- Defrost system faults: In heating mode, the outdoor coil can ice up in cold weather. The defrost cycle periodically reverses refrigerant flow to melt the ice. A failed defrost board or sensor causes the unit to lock up in ice.
- Reversing valve faults: The reversing valve switches the system between heating and cooling mode. A stuck or failed reversing valve causes the unit to only operate in one mode or not at all.
- Low ambient operation faults: Standard heat pumps lose efficiency and can fault out below about 30°F. Cold-climate (Hyper-Heating) models can operate in lower temperatures, but they have specific low-ambient fault codes.
Common Heat Pump Error Codes by Brand
| Brand / Code | Meaning | What to Do |
|---|---|---|
| Carrier: 2 flashes | High-pressure lockout | Check outdoor unit airflow; call technician |
| Carrier: 3 flashes | Low-pressure lockout | Technician required — refrigerant charge |
| Carrier: Fault 17 | Defrost control fault | Technician required |
| Trane: Code 126 | Defrost termination fault | Technician required — defrost sensor |
| Trane: Code 228 | Reversing valve fault | Technician required |
| Lennox: E11 | Defrost sensor fault | Technician required — sensor replacement |
| Lennox: E12 | Reversing valve fault — stuck or failed | Technician required |
| Rheem: 3 flashes | Low-pressure lockout / loss of charge | Technician required |
| Rheem: Code HE | High-pressure trip in heating mode | Check outdoor coil for ice; call technician |
| General: OB fault | Outdoor thermostat or ambient sensor fault | Technician required — sensor replacement |
| General: Defrost stuck | Unit running defrost cycle continuously | Technician required — defrost board |
Ice Buildup: Normal vs. Fault Condition
Seeing frost or ice on your outdoor heat pump unit in winter is normal. Heat pumps extract heat from cold outdoor air, and the coil can frost over during operation. The defrost cycle runs automatically every 30-90 minutes to clear this frost. What is not normal: thick ice buildup that covers the entire unit, ice that doesn't clear after the unit runs for several hours, or ice on the outdoor coil in summer (cooling mode).
If your heat pump is coated in a heavy layer of ice and the defrost cycle doesn't seem to be running, the defrost control board, outdoor temperature sensor, or defrost thermostat may have failed. Do not pour hot water on the unit — turn it to emergency heat mode (if available) and call a technician.
Reversing Valve Issues
A stuck or failed reversing valve is a common heat pump repair. Symptoms include: the system only blows cold air in heating mode, the system only blows warm air in cooling mode, or the system switches to the wrong mode when you change the thermostat setting. Reversing valve replacement is a refrigerant-circuit repair that requires a certified technician.
What Homeowners Can Check
Outdoor unit clearance: Ensure at least 18-24 inches of clearance around the outdoor unit. In winter, keep snow clear of the sides and top of the unit, especially the discharge area at the top.
Air filter: Replace the indoor air filter monthly during heavy use. Restricted airflow causes high-pressure faults in cooling mode and can trigger lockouts in heating mode.
Thermostat mode: Confirm the thermostat is set to the correct mode (HEAT or COOL) and the setpoint is above or below the current temperature accordingly.
Emergency heat: If the heat pump is faulting in extreme cold, switch the thermostat to Emergency Heat (EM HEAT) to run only the backup heat strips. This keeps your home warm while you wait for service.
When to Call a Professional
Heat pump refrigerant faults (low/high pressure), reversing valve faults, defrost board faults, and compressor issues all require professional diagnosis. Heat pump systems are more complex than standard air conditioners and require technicians who understand both heating and cooling cycles, refrigerant systems, and defrost controls.
Burkhardt Heating, Cooling, Plumbing & Electric installs and services heat pumps across Southeastern Wisconsin, including cold-climate heat pumps designed for Wisconsin winters. NATE-certified, 24/7 emergency service available.
Heat Pump Repair in Southeastern Wisconsin
Heat pump fault codes are your system's early warning system. When you see an error code, address it promptly — operating a heat pump with a refrigerant or defrost fault causes compressor wear that dramatically shortens system life.
If you see any of these codes, call Burkhardt at 414-206-3049. Our NATE-certified technicians can diagnose and repair the issue — usually in a single visit.
Ready to schedule service? Call Burkhardt Heating, Cooling, Plumbing & Electric at 414-206-3049 or contact us online. Available 24/7 for emergencies.
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