Lennox Furnace Error Code LED1-FAST_FLASH/LED2-SLOW_FLASH

This blink code means the furnace control board has detected that the 120V main power supply has reversed polarity, with the line and neutral conductors connected to the wrong terminals. The control board monitors incoming power polarity and locks out furnace operation when a reversal is detected. The fix requires swapping the line and neutral wires at the 120V field connection point. No parts replacement is needed in most cases.

Yes, reversed polarity is a legitimate electrical safety concern. When line and neutral are swapped, components that are intended to be de-energized when switched off may still carry voltage, creating a shock hazard and potential for equipment damage. The Lennox control board correctly identifies and locks out the furnace under this condition to prevent unsafe operation. The wiring should be corrected before restoring furnace operation.

Because this is a wiring correction rather than a parts replacement, the cost is typically limited to a service call and labor fee, generally in the range of $75 to $200 depending on your location and the technician's rates. If the reversed polarity exists upstream at a junction box or breaker panel and requires an electrician, costs could be somewhat higher. No replacement parts are needed for a straightforward polarity reversal at the furnace field connection.

Correcting reversed polarity involves working directly with 120V AC wiring, which presents a shock hazard if not handled properly. A qualified HVAC technician or licensed electrician should perform this repair. The actual correction is straightforward: the line and neutral conductors are physically swapped at the 120V field connection on the control board, but the power supply to the furnace must be shut off at the breaker before touching any wiring.

No, resetting the furnace will not clear this fault. The control board detects reversed polarity from the incoming power supply on a continuous basis, so the fault code will return immediately after any reset attempt as long as the line and neutral conductors remain swapped. The underlying wiring must be corrected first, after which the fault should clear on its own without requiring a manual reset.