Your boiler has detected that a vital temperature sensor is incorrectly fitted or has become detached, preventing the system from monitoring heat levels safely.

Mounting fault S3

What causes this fault?

This fault occurs when the S3 sensor, which monitors the temperature within the heat exchanger, is not properly seated against the pipework or has physically detached. The most common reason is a loose or damaged retaining clip, but it can also be triggered by a faulty wiring connection that prevents the boiler's computer from receiving an accurate reading.