F42 error, return temperature sensor readings are implausible

Safety first: Turn off power and let the system cool.

• Locate return NTC: Find the sensor on the return pipe or heat exchanger.
• Check contact: Ensure it is firmly clipped and in good thermal contact.
• Test resistance: Measure sensor resistance and compare with Protherm's chart. Replace if out of range.
• Inspect cable: Repair or replace damaged wiring.

Warning: Do not bypass the sensor; it is part of the safety and control system.