FC
FAULT CODE
Viessmann
Gas Valve Fault / Flue Blocked
High Severity
Engineer Required
On Viessmann boilers, code FC indicates either a gas valve fault, a faulty modulation valve control, or the flue system is blocked. Requires prompt professional attention.
Cold Weather Note
Check the flue terminal for ice blockage in cold weather before calling an engineer.
Step-by-Step Fix Guide
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Step 1
Turn off the boiler
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Step 2
Check the flue terminal externally for obvious blockages (birds, debris)
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Step 3 Engineer only
Call a Gas Safe engineer to test the gas valve and flue system
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.