Lennox Furnace Error Code LED#1 Fast Flash / LED#2 Slow Flash

This flash code indicates that the 120V main power supply wiring has reversed polarity, meaning the line (hot) and neutral conductors are connected to the wrong terminals at the furnace's field wiring connection. The control board detects this condition and locks out furnace operation as a safety measure. The fix requires swapping the line and neutral wires to their correct terminals after turning off power at the breaker.

Yes, reversed polarity is a genuine electrical safety concern and is one reason the furnace locks out rather than continuing to operate. Running equipment with reversed polarity can create shock hazards and may damage the control board or other components over time. The furnace's lockout behavior is a protective response, and the wiring should be corrected promptly by a qualified technician.

Because the repair involves only a wiring correction and no replacement parts, the cost is primarily labor. A service call from an HVAC technician to diagnose and correct this wiring issue typically ranges from $75 to $200 depending on your location and the technician's rates. If the polarity reversal originates upstream at a disconnect or electrical panel, an electrician may also need to be involved, which could add to the total cost.

Turn off power to the furnace at the circuit breaker before doing anything else. Locate the 120V field wiring connection inside the furnace cabinet and identify the line and neutral conductors. Switch their positions so each wire is connected to its correct terminal, then restore power and check whether the error clears. If you are not comfortable working with electrical wiring, a qualified HVAC technician or electrician should perform this correction.

No, resetting the furnace will not permanently clear this error because the control board will re-detect the reversed polarity condition every time power is applied. The underlying wiring fault must be corrected first. Only after the line and neutral wires are connected to their proper terminals will the error code stop appearing.