Lennox Furnace Error Code 5+4

Error code 5+4 on a Lennox furnace means the control board (model 100869) has detected reversed line voltage polarity on the 120V supply wiring. This means the hot and neutral wires are connected to the wrong terminals at some point in the circuit feeding the furnace. The furnace will shut down and display this code until the polarity is corrected, at which point the control is designed to restart automatically.

Yes, reversed line voltage polarity is a legitimate safety concern. With the hot and neutral conductors swapped, components that are supposed to be de-energized when a switch is open may remain live, creating a shock hazard during service. The furnace control board detects and flags this condition specifically to prevent unsafe operation. The wiring should be corrected before the furnace is returned to normal use.

If the fix is only a matter of swapping two wires at the furnace disconnect or control board, an HVAC technician service call typically ranges from $75 to $200 depending on your region and the technician's rates. If the reversal originates at the electrical panel and requires a licensed electrician, costs can range from $100 to $350 or more. No replacement parts are generally needed since this is a wiring correction, not a component failure.

Start by shutting off power to the furnace at the disconnect switch or circuit breaker. Then check the 120V line and neutral wiring connections at the control board and at the furnace disconnect, using a multimeter to confirm which conductor is actually carrying the hot leg. Swap the wires if they are reversed, restore power, and verify the error clears. If the reversal is traced back to the electrical panel, contact a licensed electrician to correct it.

A simple power reset will not permanently clear the 5+4 error if the underlying polarity reversal has not been corrected. According to the control board design, the unit will restart automatically once the error condition itself recovers, meaning the polarity must be corrected first. Resetting without fixing the wiring will cause the code to return immediately.