BoilerFaultCodesTechnical description
Checking wiring connections, or heating thermistor/primary thermistor
What causes this fault?
This fault occurs when the boiler's internal safety sensors detect that air is not flowing correctly through the flue system. The most common reason is a blockage or buildup of debris in the small plastic tubes or the venturi inside the boiler, which prevents the fan from safely clearing exhaust gases. In some cases, the mechanical switch that monitors this pressure has simply failed or become stuck due to wear and tear.
This fault is more frequent during periods of heavy wind or extreme cold, as high wind gusts can disrupt pressure readings and internal condensation can sometimes freeze or pool in the sensing tubes.
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.