Lennox Furnace Error Code LED1-SLOW_FLASH/LED2-FAST_FLASH

This fault code indicates a low flame signal condition where the ionization current detected by the flame sense rod has dropped below 0.7 microamps. The control board uses this signal to confirm that the burner flame is present. When the signal is too weak, the board cannot safely verify ignition and will shut the furnace down. The most common cause is a dirty or corroded flame sense rod.

The furnace is designed to shut itself down when it cannot confirm flame presence, so the immediate risk of unburned gas accumulation is controlled by the safety system. However, you should not attempt to defeat or bypass the lockout. Repeated cycling and lockouts indicate the furnace is not operating reliably, and the root cause should be corrected before depending on the unit for heating.

A reset will clear the fault code and allow the furnace to attempt ignition again, but if the underlying cause such as a dirty or failed sense rod is not corrected, the fault will return once the furnace fires and the control board measures the same low signal. Resetting without addressing the root cause typically results in repeated lockouts. Correct the sense rod condition first to achieve a lasting resolution.

In many cases, yes. The first step is to locate the flame sense rod, which is a small metalite probe positioned in the burner flame path, and clean its surface using fine steel wool to remove oxidation or residue. If cleaning does not resolve the fault, visually confirm the rod is properly positioned within the flame. If the rod is damaged or the fault persists after cleaning and repositioning, the rod should be replaced, which is a straightforward part swap for someone comfortable working on furnaces.

If the issue is resolved by cleaning the existing sense rod, there is no parts cost involved. A replacement flame sense rod typically costs in the range of $10 to $40 depending on the specific Lennox model. If a technician performs the diagnosis and repair, expect labor costs in the range of $75 to $200 depending on your local market and the time required to access the component.