Lennox Furnace Error Code E204

E204 indicates that the furnace control board has detected incoming AC line power at a frequency other than 60 Hz. Lennox furnaces require standard 60 Hz utility power to operate correctly. The furnace will not run while this condition is present. The first step is to measure the voltage and frequency at the furnace using a true-RMS multimeter.

The furnace will shut down and refuse to operate when E204 is active, which means there is no immediate fire or combustion hazard from the furnace itself. However, the underlying cause, such as a failing generator or unstable power source, could pose risks to other appliances. Restore proper 60 Hz utility power before resuming furnace operation.

If the cause is a generator or inverter producing incorrect frequency, there may be no parts cost at all, just the expense of a service call to diagnose the issue, typically in the range of $75 to $150. If the control board requires replacement, parts and labor can range from $300 to $700 or more depending on the specific board and local labor rates. A technician visit to measure and confirm line frequency is the recommended first step before purchasing any parts.

Start by measuring the frequency of the incoming power at the furnace disconnect or junction with a true-RMS multimeter. If the furnace is on a generator or backup inverter, switch to utility power or replace the power source with one that produces a stable 60 Hz output. If utility power is confirmed at the correct frequency and the fault persists, inspect wiring connections and consider replacing the control board.

Resetting the furnace may temporarily clear the fault display, but the code will return immediately if the incoming power frequency is still out of range. A lasting fix requires correcting the power source or wiring issue first. Once 60 Hz power is confirmed at the furnace, the control board should resume normal operation without the error returning.