BoilerFaultCodesPotterton E21 Fault Code
The Potterton E21 fault code means: your boiler has detected that its electrical wires are connected the wrong way round at the power source, preventing it from starting up safely.. It appears on 2 Potterton models in our database.
Technical description: Live & Neutral supply reversed. Correct the electrical supply
What causes the Potterton E21 fault?
This fault occurs when the electricity entering the boiler is wired incorrectly, with the live and neutral connections being swapped. This usually happens after electrical work has been carried out in the home or if a new fused power switch has been installed incorrectly. The boiler's safety sensors detect this 'reversed polarity' and prevent the system from firing to avoid an electrical hazard.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Potterton models that show the E21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton E21 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