U2 • Mitsubishi Electric
Fault code U2 Mitsubishi Electric: causes, diagnosis and solution
Fault code U2 indicates an abnormally high discharge temperature (TH4 > 125°C or 110°C for 5 minutes) or an excessive compressor surface temperature (TH34 > 125°C). This anomaly stops the compressor to prevent damage, with a specific restart condition for the surface temperature.
Proof status : Manufacturer reference needed for the exact range
Source: Mitsubishi Electric documentation (service manual – according to product reference)
Founder, AskMarcel.app
Founder of AskMarcel.app. Sheets are written from manufacturer documentation; the reference is shown when it has been verified.
In brief
Fault code U2 on Mitsubishi Electric means: Fault code U2 indicates an abnormally high discharge temperature (TH4 > 125°C or 110°C for 5 minutes) or an excessive compressor surface temperature (TH34 > 125°C). This anomaly stops the compressor to prevent damage, with a specific restart condition for the surface temperature. Most common cause: The main cause is compressor overheating due to insufficient refrigerant, although sensor faults (TH4, TH34), a faulty outdoor control board, or a faulty linear expansion valve can also trigger the error. First action: Measure the suction superheat and check for refrigerant leaks.. Call a registered engineer if the fault returns after basic checks or involves refrigerant work (F-Gas registered engineer required).
According to Extract 2 of the Mitsubishi Electric service manual, the compressor restart is blocked until the surface temperature drops below 95°C.
Continue this diagnosis in Marcel
Mitsubishi Electric code U2: Marcel picks this code up and guides you, with the manufacturer reference when one exists.
For regular use on site: Install the app
What does fault code U2 on Mitsubishi Electric mean?
Fault code U2 indicates an abnormally high discharge temperature (TH4 > 125°C or 110°C for 5 minutes) or an excessive compressor surface temperature (TH34 > 125°C). This anomaly stops the compressor to prevent damage, with a specific restart condition for the surface temperature.
The main cause is compressor overheating due to insufficient refrigerant, although sensor faults (TH4, TH34), a faulty outdoor control board, or a faulty linear expansion valve can also trigger the error.
Symptoms
- Compressor stops due to discharge temperature exceeding critical thresholds.
- Immediate restart impossible if compressor surface temperature (TH34) remains above 95°C.
- LED flashing on the outdoor control board (5 times, 2.5 seconds on, 2.5 seconds off).
- Compressor overheating detected due to refrigerant loss or circuit blockage.
- Persistent error if discharge temperature exceeds 110°C for 30 seconds after defrost starts.
Diagnosis
1. Check temperature parameters and refrigerant
- Measure the suction superheat and check for refrigerant leaks.
- Verify that the refrigerant charge matches the model specifications.
2. Inspect electrical and hydraulic components
- Ensure stop valves are fully open.
- Inspect connectors for thermistors (TH4, TH34) and linear expansion valve (LEV-A, LEV-B) on the outdoor control board.
- Measure the resistance across the linear expansion valve terminals.
3. Restart test and short-circuit diagnosis
- Cut the power and check if fault code U3 appears on restart (indicates a thermistor short circuit).
- Wait until the compressor surface temperature (TH34) drops below 95°C before attempting a restart.
Repair and return to service — Mitsubishi Electric fault U2
First check the integrity of the refrigerant circuit and recharge if necessary. Ensure all stop valves are open and the linear expansion valve works correctly. If connectors are disconnected or resistance values are abnormal, repair or replace the faulty components. If a thermistor short circuit occurs, fault code U3 will appear after a restart, requiring replacement of the sensor or control board.
When should you call an F-Gas engineer?
Contact an F-Gas registered engineer if the fault returns after basic checks, involves refrigerant charging, or requires PCB or compressor replacement on Mitsubishi Electric.
Affected models
- Ecodan PUHZ SW50VKA BS
- Ecodan PUHZ W50VHA BS
- Ecodan PUHZ HW140VHA2 BS
- MUYTP3550VF
Field context
This fault code occurs during load operation or defrost when the TH4 or TH34 sensors detect critical temperatures, preventing the compressor from restarting to protect the equipment from thermal destruction.
Frequently asked questions
Why does the compressor not restart after a U2 fault code?
If the error is related to the compressor surface temperature (TH34), restart is blocked as long as this sensor indicates a temperature above 95°C. Natural cooling must be allowed. If the error persists after a restart, fault code U3 may appear, indicating a thermistor fault (TH4 or TH34) requiring replacement of the sensor or control board.
What are the main causes of excessive discharge temperature?
The main causes identified are: insufficient refrigerant (leak), compressor overheating, a stop valve not fully open, a faulty thermistor (TH4), a defective outdoor control board, or a malfunctioning linear expansion valve. Blockage by foreign objects in the circuit can also occur if water has entered the system.
What procedure should be followed if fault code U2 persists after checking the refrigerant?
Check the integrity of the thermistor connections (TH4, TH34) and the linear expansion valve on the outdoor control board. Measure the resistance values across the linear expansion valve terminals. If values are abnormal or connectors are disconnected, repair or replace the faulty components. Also ensure the stop valves are fully open.
Need help with this diagnosis?
Marcel reads your error codes and walks you through them step by step, with sourced answers.
Or keep going right now in your browser:
