Furnace shows a return sensor or flow sensor error; unit may overheat or shut down prematurely.

Safety first: Isolate power and allow the system to cool before touching pipework.

• Locate sensors: On Sime hydronic furnaces, NTC sensors are usually clipped to the flow and return pipes or inserted into sensor wells near the hydraulic group.
• Check connections: Inspect the sensor plugs and wiring harness for loose connections or damage. Reseat connectors on the control board.
• Resistance check (advanced): With power off, disconnect the suspect sensor and measure its resistance with a multimeter. Compare to the temperature/resistance chart in the Sime manual. Replace if readings are far from expected.
• Replace sensor: If faulty, remove the clip or unscrew the sensor from its pocket and install a genuine Sime replacement NTC sensor, ensuring good thermal contact.