BoilerFaultCodesViessmann F5 Fault Code
The Viessmann F5 fault code means: your boiler has identified a problem with the ventilation system and has shut down as a safety precaution because it cannot confirm that exhaust fumes are being cleared correctly.. It appears on 7 Viessmann models in our database.
Technical description: Burner in fault position. The air pressure switch is open at burner start or did not close on reaching the starting load. Check the air pressure switch and the connecting hoses.
What causes the Viessmann F5 fault?
This fault occurs when the boiler's internal safety sensors fail to detect the necessary airflow required to safely clear exhaust gases. The most common cause is a blockage in the flue or a failure in the fan assembly, which prevents the system from confirming it is safe to ignite the burner.
This fault can be more frequent during periods of high winds, as strong gusts can occasionally interfere with the air pressure balance within the flue system.
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 →Viessmann models that show the F5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann F5 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026