Your boiler is working exactly as it should and is currently providing heating to your home.

HTG mode; the boiler operates in the HTG mode.

What causes this fault?

This fault occurs when the internal sensor responsible for tracking the temperature of the water leaving the boiler fails or detects an electrical short. The most common reason is that the sensor's internal components have worn out over time or the connecting wires have become damaged or loose. Because the boiler can no longer safely monitor how hot the water is, it shuts down to prevent overheating.