Open/short of outdoor unit thermistors (TH3, TH32, TH33, TH6, TH7, and TH8)

What does U4 mean?

This fault indicates an open or short circuit detected in one or more of the outdoor unit thermistors (TH3, TH32, TH33, TH6, TH7, TH8) during compressor operation. Similar to U3, detection is inoperative for a period after compressor start and during defrosting for certain thermistors. This can lead to inaccurate system monitoring and control, potentially affecting performance and component longevity.

Complete Troubleshooting Guide

1. 1

   Use the A-Control Service Tool (PAC-SK ST) and switch the mode of SW to identify which specific thermistor or unit has the abnormality. Refer to section 11-10.

2. 2

   Inspect the connections of thermistor connectors (TH3, TH32, TH33, TH6, TH7, TH8) on the outdoor controller circuit board and connector (CNF) on the outdoor power circuit board. Check for lead wire breakage. Refer to section 10-9.

3. 3

   Measure the resistance values of the affected thermistors and compare them to specifications, or check their temperature readings via microcomputer (refer to sections 10-6 and 10-10 for details).

4. 4

   If thermistors are functional and connections are secure, consider replacing the outdoor controller circuit board.