BoilerFaultCodesVaillant F20 Fault Code
The Vaillant F20 fault code means: your boiler's internal computer chips are unable to communicate properly due to a software version mismatch.. It appears on 2 Vaillant models in our database.
Technical description: MMI software incompatible with IRIS software
What causes the Vaillant F20 fault?
This fault occurs when the boiler's internal control boards are running different versions of software that cannot speak to each other. It most commonly happens after a repair where a new part has been installed that is not programmed to match the existing chips, or if a power surge has corrupted the board's memory. Because the different components cannot coordinate safely, the boiler shuts down as a precaution.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Vaillant models that show the F20 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F20 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026