High Lockout
F9 on Viessmann Vitodens 333

Your boiler's fan is not spinning fast enough to safely start the burner, causing the system to shut down for safety.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Burner in fault state. Fan speed too low at burner start. Cause: Check the fan, check the fan cables and supply, check the fan control. Press E.

What causes this fault?

This fault occurs when the boiler's internal computer tries to start the heating process but detects that the fan isn't reaching the necessary speed to clear out exhaust gases. The most common reasons are a build-up of debris causing the fan to stick, a failure in the fan's motor, or a loose electrical connection between the fan and the main control board.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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