BoilerFaultCodesPotterton A03 Fault Code
The Potterton A03 fault code means: your boiler has switched itself off because it has become too hot internally, likely due to a lack of water or a blockage preventing water from moving around the system.. It appears on 2 Potterton models in our database.
Technical description: Over temp. lock-out or circulation failure. Check pump, 3-way valve & CH water pressure. Replenish system & purge air/water via manual vent
What causes the Potterton A03 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.
DIY fix possible
Check the pressure gauge on the front of the boiler; if it is below 1 bar, top up the system using the filling loopEnsure all radiator valves are open to allow water to circulateBleed all radiators to remove trapped air and then top up the pressure againCarry out a manual reset of the boiler using the control knob
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Gas Safe Register →Potterton models that show the A03 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton A03 fault code information on this page is sourced and cross-checked against the official Potterton service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026