S.40
FAULT CODE
Vaillant
Flue Gas Recirculation Detected
High Severity
Engineer Required
The boiler has detected that flue gases may be recirculating back into the air intake. This is a safety-critical fault.
Step-by-Step Fix Guide
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Step 1
Do not attempt to inspect the flue yourself
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Step 2 Engineer only
Call a Gas Safe engineer immediately to check the flue installation and terminal
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.