Boiler indicates E11: return temperature sensor fault, which may cause the boiler to cycle rapidly or shut down.

Safety first: Isolate power before accessing internal components.

• Locate return sensor: The return NTC sensor is usually clipped to the return pipe entering the boiler or inserted into a pocket.
• Check mounting: Ensure the sensor is firmly attached and making good thermal contact with the pipe (use new thermal paste or a new clip if needed).
• Inspect wiring: Follow the cable to the control board, checking for cuts or loose plugs. Reseat the connectors.
• Measure resistance: Compare the sensor resistance to the Weishaupt temperature/resistance chart. Replace the sensor if it is out of range.

When to call a professional: If the sensor is good but the fault remains, the control board may require professional diagnosis.