BoilerFaultCodesVaillant F.93 Fault Code
Also written as F93 — same fault.
The Vaillant F.93 fault code means: your boiler has detected that the fuel/air mixture is not burning correctly, which triggered a built-in safety check to prevent inefficient or unsafe operation.. It appears on 2 Vaillant models in our database.
Technical description: Inadequate combustion quality (CO), gas family check warning
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 F.93: 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 F.93 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F.93 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 7 July 2026