Your Lennox furnace has a blinking LED on the control board, and that blink pattern is a fault code. Count the flashes, look up the code on the label inside your furnace door, and you’ll know exactly what the board is reporting. Most homeowners can do this in two minutes without tools.
Note: newer communicating models (like the SLP98 series) use a digital display with alphanumeric codes rather than LED blink patterns. The process is similar, but read the display instead of counting flashes.
Where to Find the Fault Code Chart
Open the lower access panel on your furnace. The control board is visible, and there’s a sticker on the inside of the door (or on the board itself) that lists what each blink pattern means. Count the blinks in the active burst. Some boards use a two-number sequence (like 3-4), where the first number is the flash count and the second is a qualifier.
If the sticker is missing or illegible, the Lennox product documentation portal has downloadable installation manuals organized by model number, which you can find on the rating plate inside the furnace cabinet.
Important: blink code meanings vary by model and series. The sticker on your specific unit is the authoritative reference. Generic blink-code lists from third-party sites are often wrong for your particular board revision.
Common Fault Types and What They Mean
Ignition lockout. The furnace tried to light, failed after the permitted number of attempts, and locked out for safety. The exact blink count varies by model, so check your door sticker. Common causes: dirty or misaligned flame sensor, gas valve issue, or weak spark from the igniter. The flame sensor is a thin metal rod that sits in the burner flame and gets coated with oxidation over time. A tech can clean or replace it quickly, but it shouldn’t be the first assumption — if the unit locks out again after reset, something else is driving the fault.
Pressure switch fault. The pressure switch monitors draft airflow through the system. It opens when it shouldn’t, usually because of a blocked condensate drain, a cracked or disconnected hose, or a weak inducer motor. Less commonly, it can point to a cracked heat exchanger. Don’t ignore repeated pressure switch faults. A cracked heat exchanger is a carbon monoxide risk.
High limit switch tripped. The furnace overheated and the limit switch cut it off. Most common cause: restricted airflow. Check your filter first, then make sure all your supply and return vents are open. If the filter is clean and vents are open, the problem may be a weak blower motor or a failing limit switch. The limit switch itself is a cheap part, but diagnosing why it tripped matters more than just replacing it.
Rollout switch tripped. This is a lockout you should take seriously. The rollout switch fires when flame comes out of the heat exchanger opening instead of going where it should. Causes include a cracked heat exchanger, blocked flue, or failed draft inducer. Reset it once if you want, but if it trips again, stop running the furnace and call a tech. This is a combustion safety device.
Blower timing fault. Sometimes the board reports that the blower is running when it shouldn’t, or not running when it should. This often traces to a bad control board relay or a motor capacitor starting to fail. The capacitor is a relatively inexpensive part; the board is not.
What You Can Check Yourself
A few things are worth checking before you call:
- Replace the air filter. A lot of service calls start with a clogged filter.
- Check the condensate drain. On high-efficiency furnaces, a visible backup or standing water in the drain pan is a common cause of pressure switch faults.
- Make sure all supply and return vents in the house are open. Closed vents cause high limit trips.
- Reset the furnace by cutting power for 30 seconds after you’ve confirmed there’s no gas smell and the area is safe.
Beyond that, leave it alone. The gas valve, heat exchanger, flue connections, and anything involving the inducer are not homeowner repairs. Carbon monoxide doesn’t announce itself.
How a Tech Diagnoses It
When I send a tech to a Lennox fault call, the first step is pulling the fault history off the board if the model supports it (many newer models log previous faults, which helps track down intermittent problems). Then they’ll run a full operational cycle with the service panel off, watching the ignition sequence, flame signal strength, and pressure switch behavior in real time.
Flame signal is measured in microamps with a multimeter. A weak or contaminated flame sensor will read low, and the board may shut down even if the sensor looks clean. What counts as “too low” varies by board, so the tech reads it against the spec in the service manual rather than using a universal number. Pressure switch testing involves measuring switch actuation against actual draft pressure with a manometer, not just swapping parts.
Intermittent faults, the ones that only show up sometimes, usually point to a component that’s failing under heat or load. A tech who only looks at the furnace when it’s cold and not running will miss them.
Call Before You Keep Running It
If the furnace resets and runs normally once, it’s worth watching. If the same code comes back more than once, or if you’re seeing rollout or pressure switch faults, stop running it and get a tech out.
Any fault that touches combustion, venting, or the heat exchanger is a safety issue in a sealed space with people in it. Getting it wrong costs more than the service call.
We service Lennox furnaces throughout the Tri-Valley and East Bay. If you’re seeing a fault code and aren’t sure whether it’s a filter swap or something deeper, call us. We’ll get you on the schedule fast (often same or next day when we can), tell you straight what’s wrong, and only replace what actually needs replacing. Book at adriumservice.com.