High Lockout
F20 on Vaillant ecoMAX Pro 28E

Your boiler's internal computer chips are unable to communicate properly due to a software version mismatch.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

MMI software incompatible with IRIS software

What causes this 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 is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F13View →

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