Your boiler has detected that a temperature sensor is not properly attached or is incorrectly mounted, preventing the system from heating up safely.

Mounting fault sensor S1

What causes this fault?

This fault occurs when the boiler's main flow temperature sensor has physically shifted or detached from the internal pipework. Because the sensor can no longer accurately measure how hot the water is, the system shuts down to prevent the boiler from overheating. This is often caused by a weakened retaining clip or vibration over time loosening the sensor's connection.