Abnormal Heatsink Temperature

What does U5 mean?

The heatsink thermistor (TH8) detects an abnormal temperature, either too low (below -40°F [-40°C]) or too high (above 183°F [84°C]). This indicates a potential issue with the outdoor fan, restricted airflow, excessive ambient temperature (above 114°F [46°C]), or a fault in the thermistor or associated circuit boards.

Additional Steps

Steps 1–2 are listed in Priority Fix.

1. 3

   Verify the ambient temperature around the outdoor unit to ensure it is within the specified operating limits (e.g., upper limit 114°F [46°C]).

2. 4

   Measure the resistance value of thermistor TH8 and compare it to expected values for the measured temperature.

3. 5

   If the outdoor fan, airflow, and thermistor are confirmed normal, replace the outdoor power circuit board.

4. 6

   If fan drive is faulty, replace the outdoor controller circuit board.