How to Fix / Troubleshooting

Safety first: Turn off power and allow the outdoor unit to cool. The discharge line can be very hot during operation.

Step 1 – Locate discharge sensor: Find the sensor clipped or strapped to the small-diameter copper line leaving the compressor.
Step 2 – Check attachment: Ensure the sensor is firmly in contact with the pipe. Re-secure with metal clip or high-temperature tape if loose.
Step 3 – Inspect wiring: Look for heat damage or brittle insulation on the sensor wires. Reseat the connector on the outdoor PCB.
Step 4 – Test sensor: With power off, measure resistance and compare to other NTC sensors. Replace if readings are abnormal.
Step 5 – Replace sensor: Install a new discharge temperature sensor rated for high temperature and secure it properly.
Step 6 – PCB evaluation: If F4 remains, the outdoor PCB sensor input may be faulty and should be replaced by a technician.

Display shows F4, unit may not start compressor or may shut down quickly.

Possible Causes