How to Fix / Troubleshooting

Safety warning: Disconnect power to the furnace/air handler before removing access panels or touching wiring.

1. Check breakers and switches:
- Verify the furnace/air handler breaker is ON.
- Ensure the furnace service switch (often near the unit) is ON.
2. Inspect the low-voltage fuse:
- Remove the blower door to access the Lennox control board.
- Locate the 3A or 5A automotive-style fuse and visually inspect it.
- Replace with the same rating if blown; do not upsize the fuse.
3. Verify thermostat wiring:
- Confirm R and C wires are securely connected at both the thermostat and the control board.
- Look for breaks or shorts in the cable, especially where it passes through metal.
4. Test for 24V (for qualified persons):
- With power restored, measure between R and C at the control board; you should see ~24VAC.
- If no voltage is present, the transformer or board may be failed.
5. If E210 persists after fuse and wiring checks:
- Contact a Lennox technician to test the transformer and control board and replace as needed.

Thermostat displays E210 indicating no 24V power detected on the R/C terminals; screen may be dim or rebooting.

Possible Causes