Your boiler has lost connection to the hot water cylinder sensor, meaning it cannot tell how hot your water is or when to heat it.

Short circuit in tank sensor | Cause: NTC defective, earth leak/short circuit in cable loom, damp plug connection

What causes this fault?

This fault occurs when the electrical signal from the hot water tank sensor is interrupted or fails to reach the boiler's control board. The most common reason is a physical breakage in the sensor’s internal wiring or moisture getting into the electrical plug, which confuses the boiler and prevents it from heating your water.