High
F20 on Vaillant ecoMAX Pro 18E

The internal computer systems of your boiler are having a communication mismatch, meaning the boiler has stopped working because it cannot properly coordinate its internal functions.

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

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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