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.

1. 1

   Check the connection of the TH4 connector on the outdoor controller circuit board for proper seating and check for any breakage in the thermistor's lead wire. Refer to section 10-9 for wiring details.

2. 2

   Measure the resistance value of the TH4 thermistor and compare it with specifications, or monitor its temperature reading via microcomputer using the A-Control Service Tool (refer to sections 10-6 and 10-10).

3. 3

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