Quick answer: Heat pump error codes fall into five categories — pressure faults (P1, P2 — technician required), defrost errors (dF, E4, H4), sensor errors (E1–E3, F1–F3), compressor protection errors (H6, P0 — technician required), and communication errors (E6, F6, U8). A power reset and filter cleaning resolve many non-refrigerant errors. Pressure, compressor, and reversing valve faults require a certified HVAC technician.
The Heat Pump That Stopped Heating on the Coldest Morning of the Year
It was 18°F outside and the thermostat was calling for heat when the outdoor unit stopped running and the indoor air handler displayed “E4.” I had never seen that code before. My first assumption was a refrigerant leak — an expensive repair. What I found after looking up the code in the service manual was a defrost sensor failure: a $15 thermistor clipped to the outdoor coil that had failed open circuit, preventing the defrost cycle from completing and causing the system to shut down on a protection fault.
The repair took 35 minutes and cost $15. But the more important lesson was understanding why heat pumps have error codes that other HVAC systems do not — specifically defrost errors and reversing valve errors — because heat pumps operate in both heating and cooling modes and must manage outdoor coil icing in ways that air conditioners never do. This guide to heat pump error codes covers the five fault categories unique to heat pump systems, with practical troubleshooting steps and clear guidance on which repairs require professional service.
How Heat Pump Error Codes Differ from Standard AC Error Codes
Heat pumps share most error code categories with standard air conditioners — sensor errors, airflow errors, communication errors, and compressor errors appear in both systems. But heat pumps have two additional fault categories that do not exist in cooling-only systems: defrost errors and reversing valve errors.
In heating mode, a heat pump extracts heat from outdoor air and transfers it indoors — even at temperatures well below freezing. As the outdoor coil absorbs heat from cold air, moisture in the air freezes on the coil surface. The defrost cycle reverses the refrigerant flow briefly to melt this ice, then returns to heating mode. If the defrost cycle fails — due to a sensor fault, a control board relay failure, or a refrigerant pressure issue — the outdoor coil ices over completely, blocking airflow and triggering a protection shutdown.
The reversing valve is the component that switches the heat pump between heating and cooling mode. A failed reversing valve (or its solenoid) causes the system to operate in the wrong mode — heating when cooling is called for, or vice versa — or to fail to switch modes at all. Reversing valve errors always require professional diagnosis and repair.
The 5 Heat Pump Error Code Categories
1. Pressure Faults — Low and High Refrigerant Pressure
Pressure faults are the most serious heat pump error codes. Low pressure errors (P1, E9, LP) indicate that the refrigerant charge is below the minimum operating pressure — caused by a refrigerant leak or an undercharged system. High pressure errors (P2, E8, HP) indicate that refrigerant pressure has exceeded the maximum safe operating limit — caused by a blocked outdoor coil, a failed condenser fan, refrigerant overcharge, or a failed expansion valve.
⚠️ Critical safety and legal note: All refrigerant pressure faults require a certified HVAC technician. In the United States, handling refrigerants without EPA Section 608 certification is a federal violation carrying fines up to $44,539 per day. Heat pumps use R-410A, R-32, or R-290 refrigerants — R-32 and R-290 are mildly flammable and require additional handling precautions. Never attempt to add refrigerant, check system pressure, or diagnose pressure faults without proper certification and equipment.
2. Defrost Errors — Defrost Cycle Failure or Sensor Fault
Defrost errors are unique to heat pumps and are among the most common heat pump error codes in cold climates. Common codes: dF, E4, F4, H4, C4. They indicate that the defrost cycle has failed to complete within the expected time window (typically 10–15 minutes), or that the defrost sensor cannot provide a valid temperature reading to the control board.
The defrost sensor (a thermistor clipped to the outdoor coil) tells the control board when the coil temperature has risen above freezing — signaling that defrost is complete. A failed defrost sensor (open or short circuit) causes the control board to either run defrost indefinitely or refuse to initiate defrost at all. The defrost sensor can be tested with a multimeter and replaced for $10–$25. If the sensor tests correctly but defrost errors persist, the fault is in the defrost control relay on the outdoor unit’s control board — a more complex diagnosis requiring professional service.
3. Temperature Sensor Errors — Indoor and Outdoor Thermistor Failure
Temperature sensor errors indicate that one of the system’s thermistors has failed. Heat pumps typically have more thermistors than standard AC units — indoor room sensor, indoor coil sensor, outdoor ambient sensor, outdoor coil sensor, and defrost sensor. Common codes: E1, E2, E3, F1, F2, F3 (varies by brand and sensor location).
Thermistors are NTC (negative temperature coefficient) resistors that can be tested with a multimeter. At room temperature (approximately 77°F / 25°C), most heat pump thermistors read 10kΩ–15kΩ. A reading of 0Ω (short circuit) or OL (open circuit) confirms failure. Replacement thermistors are brand-specific and cost $5–$30. Sensor replacement is a DIY-accessible repair on most heat pump systems — the sensor is typically accessible after removing the indoor unit’s front panel or the outdoor unit’s service panel.
4. Compressor Protection Errors — Overload, Locked Rotor, or Inverter Fault
Compressor protection errors indicate that the compressor has shut down on a protection fault to prevent damage. Common codes: H6, P0, C5, OL. Causes include compressor overload (drawing excessive current due to high refrigerant pressure, low voltage, or mechanical wear), locked rotor (compressor motor cannot start — failed start capacitor or seized compressor), and inverter fault (inverter board cannot drive the compressor motor — specific to inverter-driven heat pumps).
Compressor protection errors require professional diagnosis. Before assuming compressor failure, a technician will check: supply voltage (sustained undervoltage causes compressor overload), refrigerant pressure (high pressure causes compressor overload), start capacitor condition (a failed capacitor causes locked rotor on single-speed compressors), and inverter board output (for inverter-driven systems). Compressor replacement is one of the most expensive heat pump repairs — on units older than 8–10 years, full system replacement is often more cost-effective.
5. Communication and Reversing Valve Errors — Signal Loss or Mode Switching Failure
Communication errors (E6, F6, U8) indicate that the indoor and outdoor unit control boards cannot exchange signals — caused by a wiring fault in the communication cable or a failed control board. Reversing valve errors indicate that the valve that switches the system between heating and cooling mode has failed to move to the commanded position — caused by a failed reversing valve solenoid, a stuck valve spool, or a control board relay failure.
Communication errors follow the same diagnostic sequence as mini split communication errors: power reset first, then inspect the communication cable for damage or loose connections. Reversing valve errors always require professional diagnosis — the valve and its solenoid must be tested under operating conditions with the system running, which requires refrigerant pressure gauges and electrical test equipment.
Heat Pump Error Code Troubleshooting: Step-by-Step
Step 1: Perform a Power Reset
Set the thermostat to “off,” then turn off the circuit breaker for the heat pump system. For central heat pump systems, there are typically two breakers: one for the outdoor unit (30A–60A double-pole) and one for the indoor air handler (15A–20A). Turn off both. Wait 5 minutes, then restore power to the air handler first, followed by the outdoor unit. Wait 10 minutes before calling for heating or cooling — the compressor requires a pressure equalization period before restarting. If the error code returns within minutes of the reset, the underlying fault has not been resolved.
Step 2: Check the Outdoor Unit for Ice Buildup
In heating mode, inspect the outdoor unit for ice accumulation. Some frost on the coil fins is normal during cold weather — the defrost cycle clears it automatically. Heavy ice buildup covering the entire coil, the fan blade, or the unit cabinet indicates that the defrost cycle has failed. Do not attempt to chip or scrape ice from the coil — this damages the aluminum fins. Instead, turn the system off and allow the ice to melt naturally, or use a garden hose with warm water to accelerate melting. Once the coil is clear, restart the system and monitor for defrost error recurrence.
Step 3: Clean the Air Filter and Inspect the Coils
A dirty air filter restricts airflow across the indoor coil, reducing system efficiency and potentially triggering pressure or performance errors. Remove and clean or replace the filter. For central heat pump systems, also check that all supply and return vents are open and unobstructed — closed vents reduce airflow and increase system pressure. Inspect the outdoor coil for debris (leaves, grass clippings, cottonwood) and rinse with a garden hose from the inside out if needed. Ensure the outdoor unit has at least 18–24 inches of clearance on all sides.
Step 4: Test the Defrost Sensor (for Defrost Error Codes)
For dF, E4, F4, or H4 errors, locate the defrost sensor on the outdoor unit. Turn off power at the circuit breaker before accessing the outdoor unit’s service panel. The defrost sensor is a small thermistor clipped to one of the outdoor coil’s refrigerant tubes — typically near the bottom of the coil. Disconnect the sensor’s connector and measure resistance with a multimeter. At outdoor ambient temperature, compare the reading against the resistance-temperature table in the service manual. A reading of 0Ω or OL confirms sensor failure. Replacement sensors cost $10–$25 and are available by brand and model number.
Step 5: Call a Technician for Pressure, Compressor, and Reversing Valve Errors
For P1, P2, H6, P0, and reversing valve error codes, contact a certified HVAC technician. Provide the brand name, model number of both the indoor and outdoor units, and the exact error code. For refrigerant pressure errors, confirm EPA Section 608 certification before authorizing work. For compressor errors on systems older than 8–10 years, request a full system replacement cost comparison before authorizing compressor repair — the cost difference is often minimal, and a new system includes a warranty and improved efficiency.
Frequently Asked Questions About Heat Pump Error Codes
What are the most common heat pump error codes?
The most common heat pump error codes are pressure faults (P1 low pressure, P2 high pressure — refrigerant system issues), defrost errors (dF, E4, H4 — defrost cycle or sensor failure), temperature sensor errors (E1, E2, E3 — thermistor failure), compressor protection errors (H6, P0 — overload or inverter fault), and communication errors (E6, F6, U8 — indoor-outdoor unit signal loss). Codes vary significantly by brand — always verify with your specific brand and model number.
What does a defrost error mean on a heat pump?
A defrost error means the defrost cycle — which melts ice from the outdoor coil during heating mode — has failed to complete, or the defrost sensor cannot read the coil temperature. In cold weather, some frost on the outdoor coil is normal; the defrost cycle runs automatically to clear it. A defrost error indicates the cycle is not working correctly. The defrost sensor is a DIY-accessible repair ($10–$25). If the sensor tests correctly, the fault is in the defrost control relay — professional diagnosis required.
Why is my heat pump not heating and showing an error code?
The most common causes of a heat pump not heating with an error code are: low refrigerant pressure (P1 — refrigerant leak reducing heating capacity), defrost failure (outdoor coil iced over, defrost cycle not clearing it), and reversing valve fault (valve stuck in cooling mode). Low refrigerant and reversing valve faults require a certified HVAC technician. A defrost sensor error may be DIY-accessible. Always perform a power reset and check for outdoor coil ice buildup before calling a technician.
How do I reset a heat pump error code?
Turn the thermostat to “off,” then turn off both circuit breakers for the heat pump system (outdoor unit and indoor air handler) and wait 5 minutes. Restore power to the air handler first, then the outdoor unit. Wait 10 minutes before calling for heating or cooling — the compressor needs a pressure equalization period. If the error returns after the reset, the underlying fault has not been resolved and the relevant component needs to be diagnosed.
When should I call a technician for a heat pump error code?
Call a certified HVAC technician if: the error involves refrigerant pressure (P1, P2); the error involves the compressor (H6, P0, C5); the outdoor unit is heavily iced over and the defrost cycle is not clearing it after the unit is turned off; the system is not heating or cooling after a reset and filter cleaning; or the error involves the reversing valve. For refrigerant errors, EPA Section 608 certification is legally required.
Heat Pump Repair Cost by Error Code Type
The table below provides estimated repair cost ranges for the most common heat pump error code causes in the United States. DIY costs reflect the part only; professional costs include a typical service call and labor. Refrigerant and compressor repairs are professional-only.
| Error Code | Common Cause | DIY Part Cost (USD) | Professional Repair Cost (USD) |
|---|---|---|---|
| E1 / F1 — Indoor room sensor | Room temperature thermistor failure | $5 – $20 | $80 – $200 |
| E2 / F2 — Indoor coil sensor | Evaporator coil thermistor failure | $5 – $20 | $80 – $200 |
| E3 / F3 — Outdoor sensor | Outdoor ambient thermistor failure | $10 – $30 | $100 – $250 |
| dF / E4 / H4 — Defrost error | Defrost sensor or control relay failure | $10 – $25 (sensor) | $100 – $300 |
| E6 / F6 / U8 — Communication error | Communication cable or control board failure | $5 – $50 (cable) | $100 – $400 |
| P1 / E9 — Low refrigerant pressure | Refrigerant leak or undercharge | Not applicable | $200 – $800+ |
| P2 / E8 — High refrigerant pressure | Blocked coil, failed fan, or overcharge | Not applicable | $100 – $500 |
| H6 / P0 — Compressor protection | Compressor overload, capacitor, or inverter fault | $15 – $50 (capacitor) | $150 – $2,000+ |
| Reversing valve error | Reversing valve solenoid or stuck valve | Not applicable | $200 – $600 |
Note: Costs are estimates based on typical U.S. market rates as of 2024–2025. Compressor replacement on a central heat pump ($1,500–$2,500+ installed) often approaches the cost of a new system — for units older than 8–10 years, full system replacement is frequently more cost-effective and includes improved efficiency and a new warranty. Always obtain a written estimate before authorizing repairs. Refrigerant handling requires EPA Section 608 certification.
Conclusion: Heat Pump Errors Require a Different Diagnostic Mindset
The E4 defrost sensor error that shut down my heat pump on an 18°F morning cost $15 and 35 minutes to fix. But reaching that diagnosis required understanding something that most homeowners do not know: heat pumps have error codes — defrost errors and reversing valve errors — that do not exist in standard air conditioners, because heat pumps do something air conditioners cannot: extract heat from freezing outdoor air.
Heat pump error codes follow the same diagnostic logic as other HVAC systems — start with a power reset, check the filter and coils, look up the code with your brand and model number — but they require awareness of two additional fault categories. Defrost errors are often DIY-accessible (a $10–$25 sensor). Reversing valve errors always require professional service. Pressure and compressor errors are the same as in any refrigerant system: legally and practically, they require a certified HVAC technician.
The most important thing you can do when a heat pump error code appears in cold weather is to check the outdoor unit for ice buildup before assuming a refrigerant fault. A heavily iced coil that clears after the unit is turned off for 1–2 hours points to a defrost system issue — not necessarily a refrigerant leak. That distinction can save you a $200 service call and direct you to a $15 sensor replacement instead.
Sources
- U.S. Environmental Protection Agency (EPA) — Section 608 Refrigerant Management Regulations. Available at: www.epa.gov/section608
- U.S. Department of Energy (DOE) — Heat Pump Systems: Technology Overview and Efficiency Standards. Available at: www.energy.gov
- Air-Conditioning, Heating, and Refrigeration Institute (AHRI) — Industry Standards for Heat Pump Equipment. Available at: www.ahrinet.org
- North American Technician Excellence (NATE) — Heat Pump Service Technician Certification Standards. Available at: www.natex.org
- Consumer Reports — Heat Pump Reliability and Repair Cost Survey, 2023. Available at: www.consumerreports.org
