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Mitsubishi Electric Air Conditioner U3 / U4 Error Code
Outdoor temperature sensor fault. Here is what it means and how to clear it, easiest fix first.
One of the outdoor unit's temperature sensors (thermistors) has an open or short circuit.
Mitsubishi Electric lists U3 and U4 together for outdoor thermistor faults; which sensor each one points to depends on the outdoor unit series, so check its manual.
How to fix Mitsubishi Electric U3 / U4
Reset the power at the breaker
Switch off the breaker that feeds the air conditioner (some systems have separate breakers for the indoor and outdoor units, so switch off both). Wait about five minutes, switch the power back on and restart the unit to see if the code clears.
Note the full code and model number
If the code returns, write down exactly what the controller shows and the model numbers from the labels on both the indoor and outdoor units. Outdoor sensors differ between series, so this lets the technician bring the right part on the first visit.
When to call a technician
The sensors are inside the outdoor unit, so replacement is a technician job.
Parts that commonly cause U3 / U4
- outdoor thermistors
- outdoor control PCB
Order parts by your exact model number so they fit.
Other Mitsubishi Electric air conditioner codes
- Blinking OPERATION lightWall unit's indicator light is flashing
- P1Room air temperature sensor fault
- P2 / P9Indoor coil temperature sensor fault
- P4 / P5Drain sensor or drain pump problem
- PAShut down to prevent a water leak
- P6Freeze or overheat protection
- P8Pipe temperature error
- PbIndoor fan motor error
- FbIndoor control board error
- E0-E5Wall controller communication error
- E6 / E7 / E9Indoor and outdoor units aren't communicating
- U1 / UdHigh pressure or overheat protection
- U2Discharge temperature too high or low refrigerant
- U6 / UP / UFCompressor overcurrent
- U8Outdoor fan stopped
- U9 / UHPower supply or current sensor problem
- PLRefrigerant circuit abnormal
Frequently asked questions
What does U3 / U4 mean on a Mitsubishi Electric air conditioner?
One of the outdoor unit's temperature sensors (thermistors) has an open or short circuit.
Can I fix Mitsubishi Electric U3 / U4 myself?
Only the basic checks. You can safely try the steps above, but U3 / U4 usually points to a part or fault that needs a technician.
What if U3 / U4 keeps coming back?
A code that returns after a reset and the checks above usually means a part is wearing out. Note when U3 / U4 appears (which cycle, how often) and have your model number ready; it helps a technician bring the right part on the first visit.
Last reviewed 2026-10-19. Codes and steps can vary by model; check your owner's manual.