Potterton E40 fault code: what it means and what to do

On a Potterton Assure combi or system boiler, fault code E40 means the boiler has detected a short circuit or open circuit in the return central-heating NTC temperature sensor.

This meaning is specific to the Potterton Assure models listed by the manufacturer. If you have another Potterton range, check its own user guide because the same characters may have a different meaning.

What is the return heating sensor?

The return sensor measures the temperature of heating water coming back from the radiators or heating circuit. The boiler uses that reading alongside other information to control heat output safely.

When the sensor circuit is open or shorted, the boiler cannot rely on its temperature reading. It may stop operating and show E40 to prevent uncontrolled or inefficient operation.

Is Potterton E40 a low-pressure fault?

No. E40 identifies a return-temperature sensor circuit fault. Low system pressure has its own fault indication. You can check the external pressure gauge for a separate issue, but adding water will not repair an E40 sensor circuit.

What can cause fault E40?

An engineer may find a problem with the return NTC sensor, its connector, wiring or the boiler control electronics. The code does not identify which part is responsible.

Do not remove the boiler casing, unplug wiring, test electrical components or attempt to fit a sensor. These are internal appliance checks for a suitably qualified Gas Safe registered engineer.

What can you check safely?

Confirm that the boiler has power and that the external thermostat, programmer or timer is set normally. Note whether the fault followed a power cut, maintenance visit or period of intermittent heating. Also record any other code shown on the display.

Changing the thermostat temperature, repeatedly switching the boiler off and on, or topping up the system will not correct a faulty temperature-sensor circuit.

Can I reset a Potterton E40 fault?

Use only the reset instruction in the user guide for your exact model. If it permits a homeowner reset, try it once.

If E40 remains, returns or the boiler locks out again, stop resetting it and arrange a repair. A recurring sensor circuit fault needs testing, even if the boiler briefly starts again.

What will an engineer check?

The engineer will identify the exact model, review the fault history and use the relevant Potterton/Baxi service information. They may test the return-temperature sensor, connectors, wiring and control electronics. The correct repair depends on those checks; E40 does not automatically mean the sensor itself must be replaced.

Quick answer

On Potterton Assure boilers, E40 means a return central-heating NTC sensor open-circuit or short-circuit fault. Keep the casing closed and book a repair if one model-approved reset does not clear it.