Thermostat shows E12 and reports a floor sensor short or extremely high temperature reading.

Safety first: Turn OFF power at the breaker before working on sensor wiring.

Steps to diagnose:

• Disconnect sensor at thermostat: Remove the two floor sensor wires from the thermostat terminals.
• Measure resistance: Use a multimeter across the sensor leads. A reading near 0 ohms indicates a shorted cable or sensor.
• Inspect accessible cable runs: Check any exposed sections of the sensor cable for damage, staples, or pinching.
• Dry out junction boxes: If the sensor passes through a junction box, open it and check for moisture. Dry thoroughly and re-test.

If the sensor remains shorted, replace the floor sensor probe. If a new sensor still results in E12, the thermostat’s sensor input circuit may be damaged and the thermostat should be replaced.