Thermostat reports that a remote room sensor is shorted or shows an impossible temperature (e.g., -40°F or 200°F)

Safety: Turn off power to the furnace/air handler before working on any low-voltage wiring.

• Step 1 – Inspect sensor wiring: Remove the thermostat and inspect the S1/S2 sensor wires for nicks, crushed insulation, or bare copper touching.
• Step 2 – Inspect sensor location: Remove the remote sensor from the wall. Check for moisture, corrosion, or wires touching inside the back box.
• Step 3 – Resistance test: With the sensor disconnected at both ends, measure resistance across the two sensor leads. A near-zero reading indicates a short. Compare to the expected thermistor value from the sensor’s datasheet.
• Step 4 – Repair or replace cable: If the cable is damaged, cut out the bad section and splice with appropriate low-voltage cable and wire nuts, or run a new cable.
• Step 5 – Replace sensor: If the cable is good but the sensor still reads shorted, replace the remote sensor with a compatible unit.