BoilerFaultCodesTechnical description
Maximal flue gas thermostat or heat exchanger maximal thermostat has activated
What causes this fault?
This fault occurs when the water inside the boiler reaches an unsafe temperature or the flue gases become too hot, triggering a safety shut-off to prevent damage. The most common reasons are a failed internal pump that isn't moving water away quickly enough or a blockage in the system from sludge and debris. It can also be caused by an air lock or a closed valve stopping the flow of water through the heat exchanger.
This fault is more common in autumn when heating systems are switched back on after a long summer break, as pumps are prone to seizing during 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.