BoilerFaultCodesTechnical description
High limit thermostat tripped
What causes this fault?
This fault occurs when the water temperature inside the boiler exceeds a safe limit, causing the safety sensors to trigger a complete shutdown. The most common reasons are a failed pump that isn't moving water away from the heat exchanger quickly enough, or a blockage caused by a build-up of sludge and debris within the system. It can also be triggered if there is trapped air in the pipes or if the water pressure has dropped too low.
This issue is most frequently seen at the start of autumn when heating systems are turned back on after a long summer break, as pumps can often seize or stick 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.