LED flashes code 21; furnace may not respond properly to thermostat commands.
Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
Possible Causes
How to Fix / Troubleshooting
Safety: Turn off power before working on low-voltage wiring. Do not allow thermostat wires to short together.
Steps:
- Inspect thermostat wiring at furnace: Remove blower door, locate the control board, and verify that R, W, Y, G, and C wires are firmly attached and not frayed or touching each other.
- Check at thermostat: Remove thermostat from its base and inspect the same wires. Confirm that the thermostat is configured for a gas furnace (not electric heat or heat pump) per its manual.
- Bypass thermostat (test only): With power off, temporarily connect a jumper between R and W at the furnace control board. Restore power. If the furnace runs normally without code 21, the thermostat or cable is suspect.
- Inspect cable routing: Ensure thermostat cable is not stapled through or pinched, especially where it passes through walls or near sheet metal.
When to call a technician: If wiring and thermostat are correct but code 21 persists, the control board may have an internal fault and should be professionally tested and replaced.