High
F.33 on Vaillant ecoTEC plus aquaPLUS

Your boiler has stopped working because it cannot confirm that waste gases are being safely vented out of the flue.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

Main heat exchanger anti freeze protection mode due to no switching signal from the air pressure switch | Check air pressure switch, fan, fan supply or flue system

From this model's manual
Verified against the Vaillant Ecotec Plus Aquaplus installation & service manual, page 41.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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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