D.118-9 Emergency

Vaillant D.118-9 Fault Code

Also written as D1189 — same fault.

The Vaillant D.118-9 fault code means: your boiler has identified an internal electronic memory error related to its safety monitoring system, meaning it can no longer guarantee safe operation.. 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: EEPROM error
Typical repair cost £250-450 (including labour)

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.118-9 fault code

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

Source

Vaillant D.118-9 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