High
S.42 on Vaillant ecoTEC

Your boiler has detected an issue with an external add-on or a water drainage pump, which is preventing it from firing up to keep you safe.

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

Response from accessory modul or defective condensate pump is blocking burner operation

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.

Find a Gas Safe engineer
Search the official register →

Related fault codes

F.22View →F.75View →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

Frozen condensate pipe

Winter cause of ignition lockouts.

Read →