Mitsubishi Electric Mini-Split Code U3
Soft lockout
Open or short circuit of the outdoor discharge temperature thermistor (TH4) or compressor surface thermistor (TH33). Detected when the sensor reads 37F or less (open) or 422F or more (short) during compressor operation.
Manual wording: “Open/short circuit of outdoor unit temperature thermistor (TH4, TH33) - Abnormal if open (37F [3C] or less) or short (422F [217C] or more) is detected during compressor operation.”
Documented for: A-Control outdoor units (PUZ, MUZ, MXZ)
Likely causes, most common first
- Disconnection or contact failure of connectors TH4 or TH33 on the outdoor controller circuit board
- Defective thermistor
- Defective outdoor controller circuit board
What you can safely check yourself
- Turn the system off at the breaker, wait one minute, and turn it back on once
When to call a technician
Thermistor testing happens inside the outdoor unit at the control board. Call a technician.
A Mitsubishi Electric mini-split throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Service Manual No. OCH750 Revised Edition-E - PUZ-HA Outdoor Units (May 2024) (Mitsubishi Electric, PUZ-HA24NHA1, PUZ-HA30NKA, PUZ-HA36NKA, PUZ-HA42NKA1 heat-pump outdoor units) · checked 2026-08-25