U3 • Mitsubishi Electric
Fault code U3 Mitsubishi Electric: causes, diagnosis and solution
Fault code U3 indicates an open circuit or short circuit detected on the discharge temperature thermistor (TH4) and/or the compressor surface thermistor (TH34) during compressor operation. Detection is inactive for the first 10 minutes of compressor start-up and during or after defrost.
Proof status : Manufacturer reference needed for the exact range
Source: Mitsubishi Electric documentation (service manual – according to product reference)
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Founder of AskMarcel.app. Sheets are written from manufacturer documentation; the reference is shown when it has been verified.
In brief
Fault code U3 on Mitsubishi Electric means: Fault code U3 indicates an open circuit or short circuit detected on the discharge temperature thermistor (TH4) and/or the compressor surface thermistor (TH34) during compressor operation. Detection is inactive for the first 10 minutes of compressor start-up and during or after defrost. Most common cause: The main cause is a disconnection or contact failure on the connector (TH4/TH34) of the outdoor controller board, a faulty thermistor, or a faulty outdoor controller board. First action: Inspect the connector connection (TH4/TH34) on the outdoor controller board.. Call a registered engineer if the fault returns after basic checks or involves refrigerant work (F-Gas registered engineer required).
According to the Mitsubishi Electric service manual for the Ecodan PUHZ SW50VKA BS, fault code U3 is triggered by an open circuit (≤3°C) or short circuit (≥217°C) on the discharge temperature thermistor (TH4) and/or compressor surface thermistor (TH34).
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What does fault code U3 on Mitsubishi Electric mean?
Fault code U3 indicates an open circuit or short circuit detected on the discharge temperature thermistor (TH4) and/or the compressor surface thermistor (TH34) during compressor operation. Detection is inactive for the first 10 minutes of compressor start-up and during or after defrost.
The main cause is a disconnection or contact failure on the connector (TH4/TH34) of the outdoor controller board, a faulty thermistor, or a faulty outdoor controller board.
Symptoms
- Display of fault code U3 on the indoor unit.
- Compressor stops due to overheat protection or measurement fault.
- Specific blinking pattern on the outdoor unit board LED (e.g., 1 flash every 2.5 seconds on some models).
Diagnosis
1. Check connections
- Inspect the connector connection (TH4/TH34) on the outdoor controller board.
- Check the integrity of the conductor wire for the thermistor (TH4/TH34) for any breaks.
2. Measure components
- Measure the resistance value of the thermistor (TH4/TH34) or check the temperature displayed by the microprocessor.
- Use the outdoor unit monitoring function (SW2 on the A-Control service tool) to confirm the reading.
Repair and return to service — Mitsubishi Electric fault U3
If the connection and wire are intact, replace the faulty thermistor. If the resistance measurement is correct but the fault persists, replace the outdoor controller 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 ATW PUHZ W50VHA BS
- MUYTP3550VFS
- Ecodan PUHZ HW140VHA2 BS
- Ecodan PUHZ HW140YHA2 BS
Field context
This code appears during diagnosis of a Mitsubishi Electric Ecodan air conditioning fault where the compressor does not start or stops mid-cycle. The engineer must wait for the defrost cycle and the 10 minutes post-start-up to conclude a hardware fault.
Frequently asked questions
Why does code U3 not appear immediately after compressor start-up?
Detection of code U3 is intentionally inactive for the first 10 minutes of the compressor start-up process and during and after the defrost cycle. This safety measure prevents false positives due to transient thermal variations. The engineer must wait for these periods to end before carrying out resistance or continuity checks on the thermistors.
What are the exact temperature thresholds for triggering a U3 fault?
Fault U3 is triggered if an abnormal value is detected during compressor operation: an open circuit corresponding to a temperature of 3°C or less, or a short circuit corresponding to a temperature of 217°C or more. These thresholds apply to the discharge temperature thermistor (TH4) and the compressor surface thermistor (TH34/TH33).
What is the diagnostic procedure if the connector connection is correct?
If the connector connection (TH4/TH34) on the outdoor controller board and the integrity of the wires are confirmed, measure the resistance value of the relevant thermistors. If the resistance is correct but the fault persists, the cause is likely a faulty outdoor controller board, requiring its replacement. Consult the test point diagrams in the service manual.
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