Mitsubishi Electric U4

What does Mitsubishi Electric U4 mean?

The control found an open or short circuit in one of the outdoor-unit thermistors (TH3, TH32, TH6, TH7 or TH8), which sense pipe, coil, and ambient temperatures. With an unreliable sensor reading the unit stops and shows U4 on a blinking display. The problem lies in a temperature sensor or its wiring rather than the refrigerant charge.

Symptoms

• U4 shown on a blinking display
• Unit stops on the protection device
• Abnormal temperature readings in service display mode
• Fault recurs after restart

Common causes

• Loose or disconnected thermistor connector
• Open circuit from cut or pinched sensor wiring
• Shorted thermistor element
• Corrosion or moisture at a sensor connector

Diagnostic steps

1. 01

   Power down the outdoor unit

   Switch off power before opening the outdoor unit to inspect sensors.

2. 02

   Check each thermistor connector

   Verify that TH3, TH32, TH6, TH7 and TH8 connectors are seated and undamaged.

3. 03

   Inspect the harnesses

   Look for chafed, pinched, or broken wiring along each thermistor lead.

4. 04

   Dry corroded connectors

   Clean and reseat any connector showing moisture or corrosion.

5. 05

   Restart and observe

   Restore power and check whether U4 clears once connections are secure.

When to call a professional

Call a technician if U4 stays after reseating connectors, since identifying which thermistor is open or shorted requires resistance testing against the service data. Replacing a sensor inside the outdoor unit means working around the coil and control board. A qualified professional should confirm and replace the faulty thermistor.