BoilerFaultCodesVaillant S13 Fault Code
The Vaillant S13 fault code means: your boiler is struggling to light properly when you turn on your hot water taps.. It appears on 4 Vaillant models in our database.
Technical description: DHW mode ignition error
What causes the Vaillant S13 fault?
This fault occurs when the boiler detects that the burner is failing to light or stay lit specifically while trying to heat your domestic hot water. The most common reasons are a weak spark from worn-out ignition parts or a gas valve that isn't opening correctly to provide fuel. It can also be caused by moisture on the electrodes or a communication error within the core control board.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Vaillant models that show the S13 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant S13 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