Lennox furnace error code xc21

If you have a Lennox furnace, sometimes you may find that your thermostat is stuck on or the unit will not go into cooling. If this happens, the first step in troubleshooting is to diagnose what code has been generated by the furnace.

In this example, two possible codes can be generated: xc21 and xc20. The first one means an error during startup before power was applied to the heater section; if you want to fix it, turn off power at the main breaker, then restart at a breaker downstream from where it failed- this should clear up any issues with the heater. The second code means electricity has been applied, but there was an error when changing from Cool to Heat mode. This usually indicates a problem in either.

If you are experiencing this error code, it is most likely caused by faulty equipment or clogged filters. It is recommended that you have an HVAC professional inspect your home’s furnace to find a solution.

The Lennox Furnace Error Code xc21 indicates that the humidification or dehumidification function has failed and triggered an automatic reset.

The Lennox furnace error code xc21 is not technically an error code. It’s a set of diagnostics that indicate a problem with the burner in the furnace.

The Lennox furnace error code xc21 can be difficult to diagnose, therefore it is essential for a HVAC professional to know what the code means and how to handle it.


HVAC system does not turn on.

Dome light does not come on.

Dome light blinks three times when you press the switch.

A clicking noise is heard from the dome light circuit breaker or from the fan motor.

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