BoilerFaultCodesMain E40 Fault Code
The Main E40 fault code means: your boiler has detected a problem with the sensor that monitors the temperature of the water returning from your radiators.. It appears on 1 Main model in our database.
Technical description: Central heating return sensor error
What causes the Main E40 fault?
This fault occurs when the internal control board loses communication with the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the sensor itself has developed an electrical fault or the wiring connecting it to the main system has become loose or damaged.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Main models that show the E40 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Main E40 fault code information on this page is sourced and cross-checked against the official Main service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026