Faulty hot water sensor or sensor pins and plug corroded.
This is mounted on 15mm pipe inside boiler it is a small green unit with a hexagon nut. To change isolate mains water supply turn on hot tap which should stop running, turn power off to boiler remove faulty sensor and fit new one with new gasket. If the gasket on the fitting above the sensor has been leaking replace this at the same time.
Ian, September 2008
