Lennox Furnace Error Code E120

E120 indicates that the furnace's indoor control is not receiving a response from the outdoor unit during routine communication polling. It is a communication fault, not a combustion or heat exchanger failure. The cause is usually a wiring issue or a temporary lockup of the outdoor unit control board. A power recycle and connection check are the recommended first steps.

A full power recycle will clear the E120 code temporarily and may resolve it permanently if the cause was a transient control board lockup. If the underlying wiring or hardware issue is not corrected, the code will return once the system resumes polling. Monitor the system closely after a reset to see whether the fault reappears.

If the fix is simply reseating a loose connector or recycling power, there may be only a standard diagnostic service call fee, typically in the range of $75 to $150. If a communication wire must be replaced, parts and labor could add $100 to $250. An outdoor unit control board replacement is more involved, with parts and labor costs commonly ranging from $300 to $700 depending on the specific model and local labor rates.

Homeowners can safely attempt a power recycle by turning off both the indoor furnace disconnect and the outdoor unit disconnect, waiting 30 seconds, and restoring power. Visually inspecting the communication wire for obvious damage is also reasonable. However, testing control voltage and diagnosing the control boards requires electrical knowledge and a multimeter, so those steps are best left to a qualified HVAC technician.

E120 is a communication fault between system components and does not indicate a gas leak, carbon monoxide issue, or heat exchanger failure. The system typically suspends coordinated operation to prevent uncontrolled behavior, which is a protective response. While not an immediate safety emergency, the fault should be diagnosed promptly to restore proper heating or cooling function.