Lennox Furnace Error Code 5+3

The 5+3 flash code on a Lennox furnace means the control board (model 100869) has detected no earth ground on the unit. The board monitors for a valid ground connection as part of its safety logic, and when none is found, it shuts down normal furnace operation. The code appears as five LED flashes, a pause, then three flashes. The furnace will attempt to restart automatically once the grounding problem is resolved.

Yes, a missing earth ground is a genuine electrical safety concern. Without a proper ground, fault current from a short circuit has no safe return path, which increases the risk of electric shock to anyone who contacts the furnace cabinet. The control board is designed to detect and flag this condition specifically to prevent operation under unsafe electrical conditions. Do not bypass or ignore this fault.

If the fix is simply tightening or reconnecting a loose ground wire at the furnace, costs are typically limited to a service call fee, which can range from around $75 to $150 depending on your area. If new wiring needs to be run from the panel to the furnace, or if the control board 100869 needs replacement, costs can range from $150 to $500 or more, factoring in parts and labor. Getting a diagnosis from a qualified technician before purchasing parts is strongly recommended.

A knowledgeable DIYer can inspect the ground wire connections at the furnace chassis and confirm that a ground wire is present and secured. However, any work inside the electrical panel should be performed by a licensed electrician or HVAC technician, as exposed panel wiring carries lethal voltage. If the ground path checks out at the furnace end, have a qualified technician inspect the panel connection and verify continuity of the ground conductor.

A manual reset will not permanently clear this code because the control board will re-detect the missing ground and trigger the fault again. The 5+3 error is designed to recover automatically once the grounding condition is corrected, meaning no manual reset should be needed once the actual wiring problem is fixed. Repeatedly resetting the unit without addressing the ground issue will not resolve the fault.