Your boiler is struggling to detect the flow of water because of a faulty electrical connection inside the unit, meaning it will likely refuse to heat your water.

Flow sensor short circuit.

What causes this fault?

This fault occurs when the electrical component responsible for monitoring the temperature of the water leaving your boiler develops a short circuit. The most common reason is that the sensor's internal casing has failed or moisture has entered the electrical connector, causing the boiler to shut down for safety because it can no longer track how hot the water is becoming.