Mitsubishi Error Code U3: Discharge Thermistor Short/Open
Warning: Your Mitsubishi HVAC system has detected a potential issue with the discharge thermistor, which could lead to system damage if not addressed.
LED Blink Pattern
How to Fix Mitsubishi Code U3
- Turn off your HVAC system at the thermostat and the circuit breaker to prevent further system operation.
- Contact a professional HVAC technician to diagnose and repair the issue.
How to Reset Mitsubishi Code U3
- 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 U3 Mean?
The U3 error code on your Mitsubishi HVAC system indicates a problem with the discharge thermistor. This component measures the temperature of the air leaving your system, helping to maintain optimal cooling and heating. If the thermistor is shorted or open (not functioning), your system can't accurately monitor this temperature, leading to inefficient operation and potential system damage. Your system has entered a lockout mode to prevent further issues.
Common Causes
- Faulty discharge thermistor
- Wiring issue between the thermistor and the control board
- Control board failure
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| System is in lockout mode and displaying U3 error | Measure the resistance of the discharge thermistor using a multimeter | Resistance should be within the manufacturer's specified range (typically between 10,000 to 100,000 ohms, depending on the model) |
| Resistance is out of range or the thermistor is damaged | Inspect the wiring between the thermistor and the control board for any breaks, corrosion, or signs of damage | Wiring should be intact and free of 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 |
|---|---|---|---|
| Discharge Thermistor | — | — | — |
Related Codes on Same Models
Other codes that affect the same Mitsubishi model families as U3.
| 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 |