Your boiler's pilot light is on, but the main heating burner is failing to ignite because the temperature control switch is broken or incorrectly reading the heat levels.

Main burner fails to light, pilot burning - Thermostat out of calibration or faulty.

What causes this fault?

This fault occurs when the internal temperature sensor fails to signal the main burner to ignite, even though the pilot light is ready and waiting. The most common reason is that the thermostat has lost its calibration or its sensing bulb has degraded, meaning it can no longer accurately tell the boiler that the water has cooled down and needs reheating.