BoilerFaultCodesPotterton D.(1) Fault Code
Also written as D1 — same fault.
The Potterton D.(1) fault code means: your boiler is not turning off when the water gets hot enough, which runs the risk of the system overheating.. It appears on 3 Potterton models in our database.
Technical description: Main burner fails to shut down when water reaches pre-determined temperature - Thermostat out of calibration or faulty.
What causes the Potterton D.(1) fault?
This fault occurs when the internal sensors fail to tell the boiler to stop firing once the water has reached its target temperature. The most common reason is that the control thermostat has lost its calibration or become stuck in the 'on' position, meaning the burner stays lit indefinitely. This creates a safety risk as the water inside the unit could potentially reach boiling point if the safety limits do not intervene.
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 D.(1) fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton D.(1) 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