Open/Short of Outdoor Unit Thermistors

What does U3 mean?

An open or short circuit is detected in one or more outdoor unit thermistors (TH3, TH32, TH33, TH6, TH7, and TH8). These sensors are critical for monitoring various temperatures within the outdoor unit's refrigerant circuit and ambient conditions, affecting system control. Open detection for TH3, TH32, TH33, TH6 is inoperative for 10 seconds to 10 minutes after compressor start and 10 minutes after/during defrosting.

• Open circuit or short circuit detected in the outdoor unit's thermistor sensors.
• Damaged or disconnected thermistor wiring.
• Faulty thermistor (e.g., ambient, coil).
• Corrosion on sensor terminals.

Click steps to track your progress.

1. 1

   Check the connection of connectors (TH3, TH32, TH33, TH7/6) on the outdoor controller circuit board.

2. 2

   Check the connection of connector (CNAF) on the outdoor power circuit board.

3. 3

   Check the lead wires for thermistors (TH3, TH32, TH33, TH6, TH7, TH8) for any breaks or damage.

4. 4

   Measure the resistance value of each thermistor (TH3, TH32, TH33, TH6, TH7, TH8) and compare it to expected values for the measured temperature.

5. 5

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