Boiler shows F33, fan runs but air pressure switch does not confirm airflow

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Faulty air pressure switch, Blocked pressure tubes, Incorrect fan speed or blocked flue, Wiring fault to pressure switch

Safety: Isolate power and gas. Flue and combustion air components must be serviced by qualified personnel.

Steps:

Inspect and clean the silicone tubes between the fan and air pressure switch; remove any water or debris.
Check that the flue is not blocked and is correctly installed.
Test the air pressure switch with a multimeter while the fan runs; replace if it does not change state.
Verify wiring continuity between the switch and PCB.

After repair: Reset the boiler and confirm normal start‑up without F33.

Hard 4/5

Air Pressure Switch

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