BoilerFaultCodesVokera AL20 Fault Code
The Vokera AL20 fault code means: your boiler has shut down because its internal safety sensor has detected that the water temperature has risen too high.. It appears on 8 Vokera models in our database.
Technical description: Limit thermostat fault
What causes the Vokera AL20 fault?
This fault occurs when the water inside the heat exchanger reaches a dangerously high temperature, triggering a safety switch to prevent damage. The most common reasons are a pump failure that stops water from moving or a blockage in the system that prevents heat from escaping the boiler. It can also be caused by a faulty sensor sending an incorrect 'overheat' signal to the main control board.
This issue is more frequent when the heating is first turned on after summer, as pump components often seize up during long periods of inactivity.
DIY fix possible
Check that all radiator valves and boiler isolation valves are fully openTurn the selector switch to the 'OFF/RESET' position, wait five seconds, and turn it back to 'ON'Check that your system pressure is between 1 and 1.5 bar on the pressure gauge
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Gas Safe Register →Vokera models that show the AL20 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vokera AL20 fault code information on this page is sourced and cross-checked against the official Vokera service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026