Overheat Protection / Outdoor Thermistor Short/Open

What does U4 mean?

The outdoor unit has activated its overheat protection, indicating that a critical component or the overall unit temperature is too high. This could also be caused by the outdoor ambient thermistor being short-circuited or open-circuited, preventing accurate temperature sensing.

1. 1

   Verify outdoor unit fan motor operation and fan blade integrity.

2. 2

   Ensure the outdoor unit's condenser coil is clean and free of obstructions to airflow.

3. 3

   Measure the resistance of the outdoor thermistor at its leads — compare to a resistance-temperature chart if available.

4. 4

   Inspect the thermistor's wiring and connector on the outdoor unit PCB for damage or corrosion.