High Lockout
F.42 on Vaillant ecoMAX 635

Your boiler's electronic control panel is unable to identify the specific model of boiler it is supposed to be operating due to an electrical connection fault.

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

Coding resistor (in harness) malfunction

What causes this fault?

This fault occurs when the boiler's main control board loses the electrical signal used to identify exactly which model it is controlling. The most common reason is a loose or corroded connection in the wiring harness, or a failure of the internal resistor that acts as the boiler's unique identity tag.

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

F.41View →F.70View →

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