Worcester Bosch Air Pressure Switch / Flue Gas Sensor Fault

Boiler displays A4 and will not fire, often related to flue gas or air pressure sensing.

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

Blocked or kinked air pressure tubes, Faulty air pressure switch, Blocked flue, Condensate collecting in pressure tubes

Safety first: Turn off power and gas. Do not dismantle the flue unless qualified.

External flue check: Confirm that the flue terminal is clear and that any flue joints are intact and not leaking.
Condensate check: Ensure the condensate trap is not overfilled and that the condensate pipe is free-flowing.
Engineer inspection: A professional should inspect and clear the air pressure tubes, test the air pressure switch, and verify correct fan operation and flue integrity.

Professional Required 5/5

Air Pressure Switch

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