Gas valve leaking
The 20 error code on Unical boilers indicates gas valve leaking.
Unical 20 means: gas valve leaking. On this component the documented failure modes are faulty gas valve, or its wiring and connector, wiring, connector or harness of the gas valve damaged or loose or the gas valve has failed — short circuit or open circuit. You can safely reset the boiler once — if 20 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does 20 Mean
The 20 code is shown by the boiler’s control board when it detects gas valve leaking. 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 20 and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 6 different manufacturers.
| Cause |
|---|
| Faulty gas valve, or its wiring and connector |
| Wiring, connector or harness of the gas valve damaged or loose |
| The gas valve has failed — short circuit or open circuit |
| Control board (PCB) faulty |
Safety Warning
20 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.
How to Fix It
Unplug and re-seat the connector of the gas valve, then check the harness for chafing, corroded pins and breaks in continuity.
Measure the resistance of the gas valve with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range.
Only after every other check: verify the board’s supply and connections, and replace the board — fitting the correct boiler chip card or coding key where the model requires one.
Refer to the Unical service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Unical models that can display fault code 20:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Unical models | — | Possible |
Frequently Asked Questions
Unplug and re-seat the connector of the gas valve, then check the harness for chafing, corroded pins and breaks in continuity. According to the technical documentation, this is the first check on the list for 20.
20 is not classified as a high-severity fault, so the boiler can be reset once. If 20 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.