Thermostat shows E2 and may default to backup temperature or shut down heating/cooling

Safety: Turn off power to the furnace/air handler at the breaker before touching any thermostat wiring.

• Confirm remote sensor use: Check the thermostat installer settings to see if a remote indoor sensor is enabled. If no sensor is actually installed, disable the remote sensor option in the installer menu to clear E2.
• Inspect sensor wiring: Remove the thermostat from the wall and check the S1/S2 (or SEN terminals, model dependent) for loose or broken wires. Tighten terminal screws and ensure copper conductors are fully inserted.
• Check sensor resistance: At the thermostat, disconnect the sensor wires and measure resistance across the sensor pair with a multimeter. Compare to American Standard’s sensor chart (typically 10k or 20k NTC thermistor). An open (infinite) or near-zero reading indicates a bad sensor or shorted cable.
• Inspect sensor location: Ensure the remote sensor is not installed in a wet area, in direct sunlight, or in a supply register. Relocate if necessary.

Next steps: Replace the remote indoor sensor if resistance is out of spec or wiring is damaged.