Open/Short Circuit of Discharge Temperature Thermistor (TH4)

What does U4 mean?

The discharge temperature thermistor (TH4) circuit is detected as either open (temperature reading 37°F [3°C] or less) or short (temperature reading 422°F [217°C] or more) during compressor operation. This fault prevents accurate monitoring of compressor discharge temperature, impacting system protection and control. Detection is inoperative for the first 10 minutes of compressor start-up and during/after defrosting.

• Open circuit or short circuit detected in the discharge temperature thermistor (TH4).
• Damaged or disconnected TH4 wiring.
• Faulty TH4 sensor.
• Internal failure within the discharge pipe sensor.

Click steps to track your progress.

1. 1

   Check the connection of the TH4 connector on the outdoor controller circuit board for proper seating.

2. 2

   Check the lead wire for thermistor TH4 for any breaks or damage.

3. 3

   Measure the resistance value of thermistor TH4 and compare it to expected values for the measured temperature.

4. 4

   If TH4 and its wiring are confirmed correct, replace the outdoor controller circuit board.