Open/short circuit of discharge temperature thermistor (TH4)

What does U3 mean?

This fault occurs when the discharge temperature thermistor (TH4) detects an abnormal open circuit (temperature ≤ 37˚F) or short circuit (temperature ≥ 422˚F) during compressor operation. An inoperative detection period applies for the first 10 minutes of compressor start-up and during/after defrosting. This can lead to incorrect temperature readings, impacting compressor control and system efficiency, and potentially causing damage.

Additional Steps

Steps 1–2 are listed in Priority Fix.

1. 3

   If the thermistor is confirmed defective, replace it. If connections and the thermistor are sound, replace the outdoor controller board.