Abnormal power supply voltage
The U1 error code on Mitsubishi boilers indicates abnormal power supply voltage.
Mitsubishi U1 means: abnormal power supply voltage. The usual cause is sovra/sottotensione rete. You can safely reset the boiler once — if U1 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does U1 Mean
The U1 code is shown by the boiler’s control board when it detects abnormal power supply voltage. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows U1 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in the Mitsubishi service literature published by centriassistenza.org, cross-checked against the manufacturer’s fault-code table.
| Cause |
|---|
| Sovra/sottotensione rete |
Safety Warning
U1 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
Consult service manual
Refer to the Mitsubishi service documentation for the specific diagnosis procedure for fault code U1.
Check error log
Access the boiler error log to check for related fault codes that may help identify the root cause.
Contact manufacturer support
If the fault persists after basic checks, contact Mitsubishi technical support for specialist guidance.
How to Fix It
Have this checked: Sovra/sottotensione rete
Refer to the Mitsubishi service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Mitsubishi models that can display fault code U1:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Mitsubishi models | — | Possible |
Frequently Asked Questions
U1 is not classified as a high-severity fault, so the boiler can be reset once. If U1 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.