What it means
Mitsubishi's code tables group U3 and U4 together as 'open/short of outdoor unit thermistors'. The PUZ-HA manual says U4 covers several outdoor sensors and that emergency operation is possible for some of them. With a wireless remote, the self-check reports it as 3 beeps and 3 flashes in the outdoor-fault pattern. On MSZ wall units, an outdoor sensor fault shows as 6 blinks of the OPERATION INDICATOR lamp followed by a 2.5-second gap.
What to check, in order
- Write down the code
Note the code and unit number, as Mitsubishi's operation manual asks.
- Switch off and on once
Turn the system off at the breaker or isolator and back on. If U4 returns, leave it off and book a repair.
When to call an engineer
Call an air conditioning engineer (UK) or HVAC technician (US). Mitsubishi tells engineers to check the sensor connectors and wires, measure the sensors' resistance and replace the faulty sensor, or the outdoor control board if that is where the fault is.
A repair visit will usually check the sensor's reading and its wiring.
- Note the model and serial number. They're on a label on the side of the indoor unit, and on the outdoor unit.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Quick answers
What does the Mitsubishi U4 error code mean?
One of the temperature sensors in the outdoor unit is reading as short-circuited or disconnected while the compressor runs. Mitsubishi's code tables group U3 and U4 together as 'open/short of outdoor unit thermistors'. The PUZ-HA manual says U4 covers several outdoor sensors and that emergency operation is possible for some of them. With a wireless remote, the self-check reports it as 3 beeps and 3 flashes in the outdoor-fault pattern. On MSZ wall units, an outdoor sensor fault shows as 6 blinks of the OPERATION INDICATOR lamp followed by a 2.5-second gap.
Can I fix Mitsubishi U4 myself?
Engineer only (engineer visit). Start with: write down the code; switch off and on once. Call an air conditioning engineer (UK) or HVAC technician (US). Mitsubishi tells engineers to check the sensor connectors and wires, measure the sensors' resistance and replace the faulty sensor, or the outdoor control board if that is where the fault is.
Is Mitsubishi U4 worth repairing?
Worth fixing. A sensor, plug or wire fault is a routine repair. Worth fixing.
Checked against
- Mitsubishi Electric SLZ-KF09/12/15/18NA1 ceiling cassette (M-series) Technical & Service Manual TCH118A (PDF)
- Mitsubishi Electric SLZ-M15-60FA ceiling cassette Technical & Service Manual OCH522 (PDF)
- Mitsubishi Electric SUZ-M25-71VA outdoor unit Technical & Service Manual OCH684A (PDF)
- Mitsubishi Electric PUZ-HA30/36NHA4 outdoor unit Service Manual OCH504 (PDF)
- Mitsubishi Electric MSZ-FH06-18NA(2) indoor unit Service Manual OBH683E (PDF)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.