BoilerFaultCodesTechnical description
Overheat thermostat operated. Go to section ‘J’.
What causes this fault?
This fault occurs when the water inside the boiler exceeds a safe temperature, triggering a safety switch to cut power to the burner. The most common reasons are a failed pump that isn't moving water around the system or a blockage in the main heat exchanger caused by a buildup of sludge and debris. It can also happen if the system has become air-locked, preventing the heat from being carried away from the appliance.
This fault is frequently seen at the start of autumn when the heating is turned on for the first time after summer, as pump components can seize up during long periods of inactivity.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.