u4
Easy Fix
Mitsubishi Electric · Mini-Splits & Ductless Systems
Outdoor Thermistor Fault
Quick Answer
Error code U4 on your Mitsubishi Electric Mini-Splits & Ductless Systems means: Outdoor Thermistor Fault. This is usually fixable — see the step-by-step guide below.
Likely Causes
How to Fix It
1
Locate the outdoor ambient thermistor on the outdoor unit
2
Measure its resistance and compare to the Mitsubishi temperature-resistance chart
3
Check the sensor connector at the outdoor board and clean or reseat as needed
4
Inspect the sensor wire for weather damage or animal chew marks
5
Replace the thermistor if out of specification
Pro Tip
This is a common issue that most users can resolve without specialized tools. Try the first step before moving on to more complex solutions.
Parts You May Need
Outdoor ambient thermistor
Wire connectors
Sensor mounting bracket
Related Error Codes