How to Fix / Troubleshooting

Safety warning: Turn off power to the thermostat circuit before working on sensor wiring.

1. Verify sensor installation:
- Confirm that a Lennox-approved remote indoor sensor is actually installed.
- If not, disable the remote sensor option in the thermostat setup.
2. Inspect sensor wiring:
- Remove the thermostat from the wall and locate the remote sensor terminals.
- Ensure the two sensor wires are securely landed and not shorted together.
- Check the sensor connection at the remote location for loose or corroded terminals.
3. Test sensor resistance (for qualified persons):
- Disconnect the sensor and measure its resistance at room temperature.
- Compare to Lennox specifications for the sensor model; replace if out of range.
4. Replace sensor if defective:
- If wiring is intact but E221 remains, install a new Lennox-compatible remote indoor sensor.

Thermostat displays E221 and cannot read the remote indoor temperature sensor (if installed).

Possible Causes