Mitsubishi: Outdoor Unit / Compressor Protection
What this fault means
The outdoor unit reported a compressor, inverter or power fault and shut the system down (U-series codes such as U4 on Mitsubishi).
Likely causes
- Inverter/compressor protection trip
- Outdoor PCB or power-module fault
- Voltage supply problem
- Overheated outdoor unit
What you can safely check
- Replace a dirty air filter — restricted airflow causes a surprising share of furnace faults.
- Confirm registers and the outdoor unit are clear of snow, leaves and debris.
- Cycle power at the switch once and let the system retry.
Call a technician — don't keep resetting this
Outdoor Unit / Compressor Protection is a high-severity fault. Repeated resets of a safety lockout can mask a serious problem such as a cracked heat exchanger (a carbon-monoxide risk) or a failing compressor. If you smell gas, leave and call FortisBC at 1-800-663-9911 or 911 first.
Related Mitsubishi faults
Mitsubishi heat pump showing Outdoor Unit / Compressor Protection?
CANRO Heating & Cooling — same-day diagnosis across Greater Vancouver. Call 604-359-1081.