Your boiler has detected a safety fault with the temperature sensors or internal wiring that prevents it from firing up.

Fault code 20 shows (from Fault Finding Test B)

What causes this fault?

This fault occurs when the boiler's control board detects that the temperature sensors are sending an illogical signal or have disconnected entirely. It is usually caused by a physical failure of the internal thermistor or damage to the delicate wiring that connects these sensors to the main computer. Because the boiler can no longer accurately monitor how hot the water is, it shuts down as a safety precaution to prevent overheating.