BoilerFaultCodesVaillant 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 9 Vaillant models in our database.
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.
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 →Avoid the next call-out bill
A cover plan would handle faults like S.37: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
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.
- Vaillant Ecomax Condensing 600/800 series manual (PDF)
- Vaillant ecoMAX System 612 manual (PDF)
- Vaillant ecoTEC exclusive 627 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026