E11 error; the controller indicates a return temperature sensor fault.

Safety: Let pipes cool before handling sensors.

• Find the return temperature sensor clipped or strapped to the return pipe near the boiler.
• Ensure it is firmly attached with thermal paste or a tight clamp for good contact.
• Inspect the cable for mechanical damage and verify the plug is fully seated on the control board.
• If you can measure resistance, compare with Defro's NTC chart at known temperatures.

If the sensor is out of specification or intermittent, replace it with a genuine Defro-compatible NTC sensor.