BoilerFaultCodesPotterton 21 Fault Code
The Potterton 21 fault code means: your boiler has shut down because its internal safety sensors detected that it was beginning to overheat.. It appears on 1 Potterton model in our database.
Technical description: Safety Thermostat Activated (Boiler or Flue)
What causes the Potterton 21 fault?
This fault occurs when water inside the boiler reaches an unsafe temperature, triggering a safety switch to prevent damage. The most common reasons are a failed pump that isn't circulating water properly, or a buildup of sludge and limescale that blocks the heat exchanger. It can also be caused by a simple sensor failure where the boiler incorrectly thinks it is overheating.
This issue often surfaces when the heating is first turned on after summer, as pump components can seize up during long periods of inactivity.
DIY fix possible
Ensure all radiator valves are fully open to allow water to circulateCheck that the boiler pressure is between 1.0 and 1.5 bar on the gaugeAllow the boiler to cool down, then turn the selector switch to 'R' for five seconds to reset
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Gas Safe Register →Potterton models that show the 21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton 21 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