Your boiler's temperature sensor has failed or has a wiring issue, which means the system cannot safely monitor how hot the water is or provide heating and hot water.

CH flow thermistor short circuit or open circuit

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring the temperature of the water leaving the boiler fails to send a clear signal to the main control board. The most common reason is that the sensor's internal electrical resistance has broken down over time, or the wiring connecting it has become loose or damaged. Because the boiler no longer knows how hot the water is, it shuts down as a safety precaution to prevent overheating.