BoilerFaultCodesYour boiler has detected a problem with how air or water is flowing through the system and has shut down as a safety precaution.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Pressure differential switch failure (not applicable)
What causes this fault?
This fault occurs when the boiler's internal components detect that the airflow required for safe combustion is not being properly maintained. The system relies on a pressure switch to confirm the fan is moving air correctly, so a blockage in the flue or a failing fan motor will trigger this safety shutdown. In many cases, it is simply caused by debris or moisture preventing the pressure sensors from giving a clear reading.
This fault is more frequent during periods of heavy wind or extreme cold, as external debris can blow into the flue and ice can restrict the airflow necessary for the pressure switch to activate.
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 →Protect yourself from future repair bills
A boiler cover plan would cover faults like 408 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Ideal Systemiser SE is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.