BoilerFaultCodesAlpha 0A3 Fault Code
The Alpha 0A3 fault code means: your boiler has switched itself off because it has detected dangerously high temperatures, likely caused by a disruption in the flow of water circulating through your radiators.. It appears on 3 Alpha models in our database.
Technical description: Overheat thermostat lock-out. Relating to thermostats, thermal fuse, or manual reset thermostat. Check for water circulation. Check for air in the heat exchanger. Check for system restrictions and blockages. Check overheat thermostat and connections. Reset overheat thermostat if required
What causes the Alpha 0A3 fault?
This fault occurs when the water inside the boiler gets too hot too quickly, triggering a safety sensor to shut the system down. The most common reasons are a pump failure that prevents water from moving, or a blockage caused by sludge and debris trapped inside the main heat exchanger.
This issue often appears when the heating is first turned on in autumn, as pumps can seize up after being idle during the summer months.
DIY fix possible
Check that all radiator valves are fully open to allow water to circulate.Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar).Press and hold the reset button for 2-5 seconds to see if the system restarts.
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Gas Safe Register →Alpha models that show the 0A3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Alpha 0A3 fault code information on this page is sourced and cross-checked against the official Alpha service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026