Outdoor unit does not enter defrost in heat pump mode (for Armstrong Air heat pump models), H0 code or similar displayed

Safety first: Turn off power at the outdoor disconnect. Coils and refrigerant lines may be very hot or cold.

Steps:

• 1. Confirm heat pump mode: Ensure the system is actually a heat pump and is operating in HEAT mode with outdoor temperatures low enough to require defrost.
• 2. Inspect defrost sensor: Locate the defrost thermistor clipped to the outdoor coil tubing. Ensure it is firmly attached and not hanging loose. Check for damaged wiring.
• 3. Check board connections: Verify the sensor is plugged into the correct terminals on the outdoor control board.
• 4. Manual defrost test (if supported): Some Armstrong Air boards have test pins or a button to force defrost. Follow the service label instructions. If the unit will not enter defrost, the board may be faulty.
• 5. Replace sensor if out of range: With power off, measure sensor resistance and compare to the manufacturer’s chart. Replace if readings are incorrect.
• 6. If H0 persists: The defrost control portion of the board may be defective and require replacement by a technician.