BoilerFaultCodesWhat the manual says
Manufacturer manualOver temp. lock-out or circulation failure. Check pump, 3-way valve & CH water pressure. Replenish system & purge air/water via manual vent
What causes this fault?
This fault occurs when the water inside the heat exchanger reaches a dangerously high temperature, triggering a safety shutdown to prevent damage. The most common reasons are a pump that has failed to move water around the system or a blockage in the pipework that is trapping heat inside the boiler.
This fault often appears during the first cold spell of autumn when the heating is turned on after a long summer break, which can cause internal pump components to seize.
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.
Avoid the next call-out bill
A cover plan would handle faults like A03: no call-out fees, no surprise bills.
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