Furnace display indicates return temperature sensor fault; unit may run at reduced output or shut down

Safety: Turn off power before accessing the control compartment or piping.

• Locate sensor: Find the IVT return temperature sensor clipped or inserted into a well on the return water pipe.
• Check connection: Ensure the sensor is fully seated in the pipe pocket and the retaining clip is secure. Inspect the cable for cuts or kinks.
• Test continuity: With power off, disconnect the sensor from the control board and measure resistance with a multimeter. Compare the reading to the IVT temperature/resistance chart. An open circuit or zero ohms indicates a bad sensor.
• Replace if faulty: If readings are out of range, replace the sensor with an IVT-compatible NTC sensor and reconnect to the same terminals.

If a known-good sensor still triggers the error, the control board input may be defective and should be evaluated by a technician.