D.118-5 Emergency

Vaillant D.118-5 Fault Code

Also written as D1185 — same fault.

The Vaillant D.118-5 fault code means: your boiler has detected an internal issue with its safety monitoring sensor, which prevents it from operating to ensure your home remains safe.. It appears on 2 Vaillant models in our database.

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

Technical description: CO sensor fault: Sensor short circuit/open circuit

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Vaillant models that show the D.118-5 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant D.118-5 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