BoilerFaultCodesTechnical description
Overheat thermostat lock-out. Relating to thermostats, thermal fuse, or manual reset thermostat. Check for water circulation. Check for air in the heat exchanger. Check for system restrictions and blockages. Check overheat thermostat and connections. Reset overheat thermostat if required
What causes this fault?
This fault occurs when the water inside the boiler gets too hot too quickly, triggering a safety sensor to shut the system down. The most common reasons are a pump failure that prevents water from moving, or a blockage caused by sludge and debris trapped inside the main heat exchanger.
This issue often appears when the heating is first turned on in autumn, as pumps can seize up after being idle during the summer months.
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.