What does error code U3 mean on a Mitsubishi Electric mini-split?

Outdoor thermistor open/short. An outdoor unit thermistor (temperature sensor) is open or shorted.

Can I fix Mitsubishi Electric code U3 myself?

Start with these safe checks: Turn the system off at the breaker for a few minutes and back on. If it returns, call an HVAC technician.

When should I call a technician for code U3?

If it returns, call an HVAC technician. Don't open the sealed refrigerant system, cut or braze lines, or add refrigerant. That needs an EPA-certified technician. Turn power off at the breaker or disconnect before opening any panel, and leave capacitors, compressors and control boards to a pro: they can hold a dangerous charge even with the power off.

Mitsubishi Electric Mini-Split Error Code U3 / U4: Outdoor…

What U3 means

An outdoor unit thermistor (temperature sensor) is open or shorted.

Also shown as: U4, depending on the model.

Likely causes

Thermistor or wiring fault

Safe first checks

Turn the system off at the breaker for a few minutes and back on.

When to stop

If it returns, call an HVAC technician.

Checked against: Mitsubishi Electric, Mr. Slim indoor/outdoor unit malfunction-diagnosis check codes (2024-11-18). Codes can differ by model and year, so confirm with your owner's manual.