BoilerFaultCodesPotterton A40 Fault Code
The Potterton A40 fault code means: your boiler is struggling to move water through the system properly, which is causing it to shut down to prevent overheating.. It appears on 1 Potterton model in our database.
Technical description: System response fault. Check pump operation and speed setting. Check 3-way valve operation. Purge air from H/E. Check by-pass and other water valves open. Check boiler control responds to flow & ret. sensors
What causes the Potterton A40 fault?
This fault occurs when the boiler detects that water isn't circulating quickly enough to carry heat away from the burner, leading to a risk of internal damage. The most common reasons are a pump that has seized or slowed down, a blockage in the central heating pipes, or a diverter valve that is stuck and preventing water flow. Essentially, the boiler is 'choking' on its own heat and shuts down as a safety precaution.
This issue often surfaces in autumn when the heating is turned back on after a long summer break, as internal moving parts like the pump or valves can seize up from 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 A40 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton A40 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