Boiler shows E20; return temperature sensor reading is implausible.

Safety: Isolate power and allow the boiler to cool.

• Locate return NTC: Find the return temperature sensor on the return pipe or heat exchanger manifold of the CTC boiler.
• Check mounting: Ensure the sensor is firmly attached to the pipe or fully inserted into its pocket with thermal paste if specified.
• Inspect wiring: Check the harness from the sensor to the PCB for damage or loose connectors.
• Measure resistance (technician): Test the NTC with a multimeter and compare with CTC's resistance chart. Replace the return NTC if readings are incorrect.