Mitsubishi Electric Mini-Split Code U4
Soft lockout
Open or short circuit of one of the outdoor unit thermistors (TH3 liquid, TH32 suction, TH6 two-phase pipe, TH7 ambient, TH8 heatsink), detected during compressor operation.
Manual wording: “Open/short of outdoor unit thermistors (TH3, TH32, TH6, TH7, and TH8) - Abnormal if open or short is detected during compressor operation.”
Documented for: A-Control outdoor units (PUZ, MUZ, MXZ)
Likely causes, most common first
- Disconnection or contact failure of the thermistor connectors on the outdoor controller circuit board or connector CN6 on the outdoor power 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
Identifying which of the five outdoor sensors failed requires the service tool and resistance measurements inside the outdoor unit. 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