S.37 Medium

Vaillant S.37 Fault Code

Also written as S37 — same fault.

The Vaillant S.37 fault code means: your boiler is temporarily pausing because the fan speed is not within the required range for a safe startup.. It appears on 10 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Fan unit waiting period

What causes the Vaillant S.37 fault?

This fault occurs when the boiler's internal computer detects that the fan isn't spinning at the correct speed required to safely clear exhaust gases before ignition. The most common reasons are a buildup of dust or debris within the fan assembly, or a communication error between the fan and the main control board. If the fan cannot reach its target speed, the boiler will temporarily pause as a safety precaution.

This issue can occasionally surface during very windy weather, as extreme gusts can create back-pressure in the flue that interferes with the fan's startup speed.

Typical repair cost £150-350 (including labour)

Engineer required

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

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Vaillant models that show the S.37 fault code

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

Source

Vaillant S.37 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