Temperature sensor
Common error-code part. Confirm resistance and connector style.
Carrier Heat Pump E5 Error Code — What It Means
Carrier heat pump error code E5 indicates a defrost fault — the heat pump’s defrost cycle either failed to initiate, failed to complete, or the outdoor coil thermistor detected an abnormal temperature condition. E5 appears on Carrier mini-split and ductless heat pump displays (models like the Carrier Performance 40MAQB, 40GRB, and Infinity heat pump systems). On some split-system heat pumps, E5 may also appear on the thermostat or the air handler display.
Common Causes
- Defrost sensor (outdoor coil thermistor) fault — The thermistor that initiates defrost by detecting low outdoor coil temperature has failed or disconnected. If the sensor reads out of range, the defrost sequence can’t start or can’t terminate properly.
- Refrigerant charge low — Low refrigerant causes the outdoor coil to run abnormally cold during heating mode, causing excessive ice buildup that defrost can’t fully clear. The control detects prolonged defrost conditions and faults.
- Outdoor fan motor not running during defrost — During defrost, the outdoor fan stops and the reversing valve shifts to cooling mode to push hot refrigerant to the outdoor coil. If the reversing valve fails to shift, the coil won’t warm during defrost.
- Reversing valve fault — A stuck reversing valve (solenoid coil failure or mechanical sticking) prevents the heat pump from entering or exiting defrost mode properly, triggering E5.
Step-by-Step Fix
- Check for visible ice on the outdoor unit — Inspect the outdoor coil fins. Heavy ice accumulation (more than 1/4 inch thick coating the entire coil) indicates defrost is failing. Mild frost on the bottom portion of the coil is normal during heating operation.
- Manually defrost the unit — Turn the system to cooling mode briefly (even in winter) to run the reversing valve and force the outdoor coil to warm. Alternatively, turn the unit off and allow natural thaw.
- Check the outdoor coil thermistor — The thermistor is a small sensor clipped to the outdoor coil fins, connected to the outdoor PCB. Unplug and measure resistance — typical Carrier coil thermistors read 5–15 kΩ at 32–77°F. Readings outside this range mean sensor replacement.
- Verify refrigerant charge — If E5 occurs consistently in heating mode in mild weather (above 25°F), low refrigerant is likely. A technician needs to check system pressures with gauges.
- Test the reversing valve — With the system in heating mode, verify the reversing valve solenoid coil has 24V AC when cooling is commanded. If voltage is present but the valve doesn’t shift (can be heard as a click), the valve itself may be stuck.
Parts Often Needed
| Part | Notes |
|---|---|
| Outdoor coil thermistor | Carrier 338818-702 or model-specific |
| Reversing valve solenoid coil | 24V AC solenoid coil |
| Reversing valve (if stuck) | Mechanical replacement — requires brazing |
Technician Tips
Carrier uses different diagnostic systems across furnaces, heat pumps, chillers, mini-splits, VRF equipment, rooftop units, and communicating controls. Confirm the full model number and whether the fault came from the indoor board, outdoor board, thermostat, or equipment controller before matching it to a repair path.
Start with power, wiring, airflow or water-flow, sensor, condensate, and safety-circuit checks before ordering control boards. If the same code returns after one reset and the basic cause is not obvious, test the named circuit instead of repeatedly cycling the equipment.
Parts note: For Carrier heat-pump defrost faults, prioritize the outdoor coil sensor, ambient thermistor, defrost board, reversing-valve coil, and fan components before considering larger replacements.
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
Low refrigerant charge and reversing valve replacement require an EPA 608-certified HVAC technician. Don’t ignore E5 — running a heat pump with a failed defrost system causes ice to accumulate and can damage the outdoor fan blade and coil.