Mitsubishi Error Code U4: Outdoor Thermistor Short/Open
Warning: Your Mitsubishi HVAC system has detected an issue with the outdoor thermistor, which may affect its performance and efficiency.
LED Blink Pattern
How to Fix Mitsubishi Code U4
- Check for any visible damage or debris around the outdoor unit and remove if present
- Ensure that the outdoor unit is properly grounded and that all electrical connections are secure
- If the issue persists, contact a professional HVAC technician for further diagnosis and repair
How to Reset Mitsubishi Code U4
- Turn off the unit using the remote control or wall controller.
- Turn off the circuit breaker for the outdoor and indoor units. Wait 60 seconds.
- Restore power at the breaker.
- Turn the unit on with the remote control. If the error returns, the fault needs professional diagnosis.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does Mitsubishi Error Code U4 Mean?
The U4 error code on your Mitsubishi HVAC system indicates a short or open circuit in the outdoor thermistor. The thermistor is a temperature-sensing component that plays a crucial role in maintaining your system's optimal performance. When it malfunctions, your HVAC system may not accurately sense the outdoor temperature, leading to incorrect operation and potential comfort issues.
Common Causes
- Damage or deterioration of the outdoor thermistor due to age or environmental factors
- Faulty wiring or connections between the thermistor and the system's control board
- Interference from nearby electrical devices or power surges
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| The outdoor unit is not running | Measure the resistance of the thermistor using a multimeter | Resistance should be within the manufacturer's specified range (typically between 10,000 and 100,000 ohms) and change with temperature |
| The outdoor unit is running but the system is not functioning properly | Inspect the wiring and connections between the thermistor and the control board for any signs of damage or corrosion | Secure and intact wiring with no visible damage |
Applicable Mitsubishi Models
- City Multi (VRF)(electric)
- City Multi (Y)(electric)
- Diamond (PJ)(electric)
- Diamond Plus(electric)
- Diamond Series(electric)
- Ducted Systems(electric)
- M-Series(electric)
- MSZ(electric)
- MSZ Series(electric)
- MSZ-AP(electric)
- MSZ-AP Series(electric)
- MSZ-AP5 Series(electric)
- MSZ-AS(electric)
- MSZ-FD(electric)
- MSZ-FE(electric)
- MSZ-FS(electric)
- MSZ-FS/FD Series(electric)
- MSZ-FT(electric)
- MSZ-FY(electric)
- MSZ-GE(electric)
- MSZ-LV(electric)
- MUZ-FE Series(electric)
- P-Series(electric)
- PUMA(electric)
- PUMY(electric)
- Puff (P-Series)(electric)
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Outdoor Thermistor | 5401A000000 | — | Find Part |
Related Codes on Same Models
Other codes that affect the same Mitsubishi model families as U4.
| Code | Description | Severity |
|---|---|---|
| 1 | Indoor/Outdoor Mismatch | Warning |
| 2 | Power Supply Phase Error | Warning |
| 4 | Inverter Error | Warning |
| 7 | Outdoor Freeze/Overheat | Warning |
| E6 | Indoor/Outdoor Communication Error | Warning |
| E9 | Indoor/Outdoor Communication Fault | Warning |
| P2 | Indoor Pipe Thermistor Error | Warning |
| P4 | Drain Sensor Error | Warning |
| P6 | Coil Freeze/Overheat Protection | Warning |
| U2 | Low Voltage/Overcurrent | Warning |