Mitsubishi Electric U3 Error Code

What does Mitsubishi Electric U3 mean?

The control has detected an open or short circuit in the discharge-temperature thermistor (TH4, TH33) on the outdoor unit. Because the controller can no longer trust the discharge temperature reading, it stops the compressor and shows U3 on a blinking display. The fault is in the sensor or its wiring, not necessarily in the refrigerant circuit.

Symptoms

• U3 displayed on a blinking display
• Compressor stops on the protection device
• Fault appears even when the unit otherwise runs normally
• Erratic or missing discharge-temperature readings in service mode

Common causes

• Disconnected or loose TH4/TH33 thermistor connector
• Damaged or pinched thermistor wiring (open circuit)
• Shorted thermistor element
• Moisture or corrosion at the sensor connector

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Diagnostic steps

1. Power down safely

   Turn off the outdoor unit before inspecting any wiring or connectors.

2. Inspect the thermistor connector

   Check that the TH4/TH33 discharge thermistor is plugged in and the connector is clean and seated.

3. Look for wiring damage

   Trace the thermistor harness for cuts, pinches, or chafing that could open or short the circuit.

4. Check for moisture

   Dry and reseat any connector showing corrosion or water ingress.

5. Restart and confirm

   Restore power and verify whether U3 clears; a persistent code points to a failed thermistor.

When to call a professional

Call a technician if U3 persists after the connector is reseated, or if the thermistor itself must be tested or replaced. Measuring thermistor resistance and confirming an open or short requires a meter and the service data. Sensor replacement on the outdoor unit involves working near the compressor and electrical board and is best left to a qualified professional.