BoilerFaultCodesPotterton C(4) Fault Code
Also written as C4 — same fault.
The Potterton C(4) fault code means: your boiler's temperature control dial is no longer reading the heat correctly, causing the system to shut down or behave inconsistently for safety.. It appears on 3 Potterton models in our database.
Technical description: Thermostat out of calibration or faulty
What causes the Potterton C(4) fault?
This fault occurs when the mechanical sensing bulb inside the thermostat fails to accurately gauge the temperature of the water. Over time, the internal components can wear out or lose calibration, meaning the boiler can no longer safely determine when to turn the burner off. This leads to the system shutting itself down to prevent the water from reaching dangerously high temperatures.
This issue often becomes apparent in autumn when the heating is first switched back on after summer, as the moving parts within the thermostat can seize up through lack of use.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Potterton models that show the C(4) fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton C(4) 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