Lennox furnace with a SureLight ignition control fault codes (G61MP, G51MP, SLP98, EL296, ML195, SureLight / Basic SureLight boards)
Every fault code for this Lennox furnace with a SureLight ignition control is below, with what it indicates and a verdict: whether you can safely check it or it is a call for a technician. Here the code is the combination of two LEDs. The source is Lennox's own service manual. Confirm your model is on the list; other Lennox controls use different codes.
Before any code: if you smell gas or a carbon monoxide alarm is sounding, this is not a diagnostic exercise. Leave the house and call from outside. What to do about a gas smell or a CO alarm.
How to read the display
Lennox SureLight boards do not use a flash count. They have TWO diagnostic LEDs, and the code is the combination of what each one is doing — a slow flash (one per second), a fast flash (three per second), steady on, steady off, or alternating. So a code is read as a pair, for example "LED 1 slow flash, LED 2 steady on". Identify which LED is which from the board layout in the furnace's installation instructions before reading a code, because reversing them reverses the meaning. Lennox calls a one-hour safety lockout a "Watchguard". This applies to furnaces using the SureLight or Basic SureLight control; confirm your board against the label inside the furnace.
What each code means
Each code carries a safety verdict. Caution means there is a check you can safely make; pro-only means the correct repair is certified work and we do not walk through it here.
LED 1 slow / LED 2 on — Primary or secondary limit open
Primary or secondary limit open
A limit switch is open. Depending on the board, the limit must close within a few ignition trials or within three to five minutes, or the board drops into a one-hour limit Watchguard. As with any limit trip, the usual root cause is not enough air moving across the heat exchanger.
What you can do: Replace a dirty filter and open any blocked vents — restricted airflow is the common cause and the one you can address. If it keeps tripping with a clean filter, the limit or blower needs a technician.
LED 1 off / LED 2 slow — Pressure switch open or blocked venting
Pressure switch open or blocked venting
The pressure switch is open or opened five times in one call for heat. The manual's listed causes are a blocked inlet or exhaust vent, a blocked condensate line, or a pressure switch that was closed before the combustion-air blower started.
What you can do: You can safely clear the outdoor intake and exhaust terminations of snow, leaves or a nest. The condensate line and the switch itself are a technician's diagnosis.
Both alternating slow — Watchguard — burners failed to ignite
Watchguard — burners failed to ignite
Both LEDs alternating in a slow flash is the ignition Watchguard: the burners failed to light and the board has locked out for the safety period.
What you can do: Confirm the gas is on and the filter is clean, then let it retry or reset once at the thermostat. If it will not light, it is an ignition, gas or flame-sensing fault for a technician.
LED 1 slow / LED 2 off — Flame sensed without the gas valve energized
Flame sensed without the gas valve energized
The board detected flame when the gas valve should have been closed. Because it can mean a gas valve that is not shutting off, it is not a wait-and-see condition.
What you can do: Call a technician. If you also smell gas, leave and treat it as the emergency at the top of this page.
LED 1 on / LED 2 slow — Rollout switch open, or a loose control connector
Rollout switch open, or a loose control connector
The rollout switch is open, or the main control connector is not seated. A tripped rollout means burner flames were not being drawn into the heat exchanger properly — a combustion safety trip, not a nuisance.
What you can do: Do not keep clearing a rollout to run the furnace. It points at a venting or combustion fault a technician must find before the furnace runs again.
LED 1 fast / LED 2 slow — Reversed line-voltage polarity
Reversed line-voltage polarity
Line and neutral are reversed. The manual's instruction is simply to switch line and neutral, but that is line-voltage wiring.
What you can do: Line-voltage wiring is for an electrician or the installer, especially right after an installation or service call.
LED 1 slow / LED 2 fast — Low flame signal
Low flame signal
The flame-sense current has dropped below the level the board trusts. The manual's remedy is to replace the flame-sense rod, which is a combustion-side part.
What you can do: Servicing the flame-sense rod is combustion work this site does not walk through. It is usually a short technician visit.
Both alternating fast — Bad ground, low voltage, or a failed igniter
Bad ground, low voltage, or a failed igniter
Both LEDs alternating fast points at an improper main ground, line voltage below 75 volts, or a broken or open igniter circuit. The board will not ignite safely until the electrical fault is resolved.
What you can do: Grounding, supply voltage and the igniter circuit are a technician's diagnosis, not a homeowner adjustment.
LED 1 on / LED 2 on — Circuit board failure or miswired control
Circuit board failure or miswired control
Both LEDs steady on (and the related steady on/off and off/on combinations) indicate a circuit board failure or a control that has been wired incorrectly.
What you can do: Diagnosing a failed board versus a wiring error is a technician's job, and a good one rules out the wiring before condemning the board.
A note on brand names
We use the Lennox name only to identify which equipment these codes apply to. We are not affiliated with Lennox and are not an official support channel, and the code map above is written in our own words from the published service manual cited below.
Sources (1)
- Lennox Industries — SureLight and Basic SureLight Ignition System — Service Literature — Diagnostic Codes, Table 1 (Corp. 9710-L7, Rev. 9-2003) (accessed 2026-08-21)