BoilerFaultCodesTechnical description
Overheat thermostat or Flue thermostat operated. Turn selector to reset; if it does not extinguish, check thermostats or sections H and K.
What causes this fault?
This fault occurs when the water inside the boiler reaches an unsafe temperature or the exhaust gases become too hot, triggering a safety sensor to cut the power. The most common reasons are a pump failure preventing water from circulating or a blockage within the heat exchanger that traps heat inside the unit. It can also be caused by a build-up of air pockets or various faulty internal sensors failing to accurately read the temperature.
This issue is more frequent during the first cold snap of autumn when a pump that has been idle all summer suddenly seizes or struggles to move cold, heavy sludge through the system.
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.