Lennox Furnace Blink Codes: 2–9 Flashes (SureLight® Diagnostic Guide)

Common Symptoms

• Control board LED blinking in a repeated counted pattern visible through the sight glass
• Furnace not producing heat — shuts off after inducer starts
• Furnace attempts ignition and locks out repeatedly
• Inducer motor runs but no ignition sequence starts
• Furnace starts and runs briefly then shuts down mid-cycle
• Furnace completely unresponsive with LED blinking fast continuously

Most Likely Causes

1. 1

   2 Blinks — Pressure Switch Stuck Closed

   The pressure switch closed before the inducer motor started, which tells the board the switch has failed in the closed position or is stuck. A pressure switch stuck closed can be caused by a switch with contacts welded together, a waterlogged pressure hose that holds residual suction, or on 90+ efficiency Lennox models, condensate backup in the collector box that keeps pressure applied to the switch port even at rest. The board will not start the inducer or proceed to ignition while the pressure switch is showing closed at startup.

2. 2

   3 Blinks — Pressure Switch Stuck Open

   The draft inducer ran but the pressure switch did not close within the required timeframe. Lennox uses single-port pressure switches on most models — the switch monitors inducer draft by sensing a negative pressure (vacuum) signal from the inducer housing. Causes: a kinked, cracked, or condensate-blocked pressure hose, an inducer motor not building adequate draft (worn bearings, dirty housing, cracked wheel), a clogged secondary heat exchanger drain on 90+ efficiency models, or a failed pressure switch. On EL296V and SLP98V two-stage models, confirm both pressure switch hose connections are intact — high-stage and low-stage can have separate pressure taps.

3. 3

   4 Blinks — Open High-Limit Device

   A high-temperature limit switch has opened because the heat exchanger reached an unsafe temperature. The furnace shuts off and will not restart until the limit cools and resets automatically (or is manually reset if it is a manual-reset rollout type). Primary cause: clogged air filter blocking airflow over the heat exchanger. Secondary causes: blocked return air, closed supply registers, a blower motor running below rated speed due to a failing capacitor, or a failed blower motor. On Lennox EL180 and ML180 series, check the blower wheel for accumulated dust — a coated wheel reduces airflow significantly without any obvious filter restriction.

4. 4

   7 Blinks — Gas Valve / Ignition Failure

   The furnace attempted the full ignition sequence — inducer on, pressure switch closed, igniter energized — but the gas valve did not open or did not sustain a flame through the trial for ignition period. Causes: a gas valve coil open or shorted (test resistance across valve terminals — Lennox valve coils typically read 20–60 ohms), a gas supply issue (manual shutoff partially closed, low utility pressure), or an igniter that glows but the gas valve is not receiving the 24VAC signal from the board. The 7-blink code is the key failure path on Lennox furnaces — more specific than a generic ignition lockout code.

5. 5

   8 Blinks — Low Flame Sense Signal

   The furnace ignited — flame established during the trial period — but the flame sensor current through the rectification circuit is below the threshold needed to keep the gas valve open. The furnace lights briefly then shuts off and eventually locks out. Almost always caused by an oxidized flame sensor rod. The white or gray oxide coating on the sensor tip reduces the 1–2 microamp rectification signal below the board's minimum threshold. Cleaning the sensor rod with fine emery cloth resolves the majority of 8-blink codes on Lennox furnaces.

6. 6

   9 Blinks — Igniter Failure

   The Lennox SureLight® silicon nitride igniter did not confirm proper operation during the igniter warm-up period. The board energizes the igniter and monitors current draw — if the expected current draw is not detected within the warm-up time, 9 blinks fires. Causes: a cracked or open-circuit SureLight® igniter (the silicon nitride element is fragile and can crack from mechanical contact or thermal stress), a failed 120V supply to the igniter circuit, or an igniter connector with a broken pin. Part numbers: LB-100549G and 73K9701 are the OEM Lennox SureLight® igniters. H8908A is an alternative Lennox igniter part used on some model series.

Quick DIY Checks

1. 1Count the blink code: stand at the front of the furnace and look through the sight glass (the small rectangular window on the lower access door). Count the number of LED flashes in a complete sequence before the pause, then confirm with a second full cycle. The LED blinks the code, pauses approximately 2–3 seconds, then repeats the same count continuously. A slow single blink at roughly one flash per second with no pause is normal standby. A rapid continuous flash (multiple flashes per second) indicates normal heat call operation. Write the count down before opening the panel or touching anything.
2. 2For 3-blink pressure switch open: locate the small rubber pressure hose — typically 1/4-inch diameter — that runs from the inducer motor housing to the pressure switch. On 90+ efficiency Lennox models (EL296V, SLP98V, SL297NV), the hose may route from the secondary heat exchanger collector box. Trace the hose for kinks, soft spots, or white calcium deposits inside the tube. Disconnect both ends and blow firmly through the tube to clear any condensate or debris. Reconnect and cycle the furnace. If 3 blinks returns, test the Lennox single-port pressure switch with a multimeter in continuity mode: disconnect the hose from the switch port and apply firm suction with your mouth to the port — the switch contacts should close and beep continuity. No beep means the switch has failed. Part number 62J04 is a common Lennox replacement pressure switch.
3. 3For 4-blink high limit open: replace the air filter before anything else — a clogged filter is the leading cause of Lennox high-limit trips. After replacing the filter, allow 20–30 minutes for the heat exchanger to cool (the limit auto-resets when temperature drops below its set point). Call for heat and watch the blower. The Lennox blower should start within 30–60 seconds of the burners lighting and should run at full speed within 90 seconds. If the blower sounds sluggish or doesn't come up to speed, test the blower run capacitor with a multimeter in capacitance mode — a capacitor reading more than 10% below its rated microfarad value needs replacement. On EL180 and ML180 series furnaces, the blower wheel accumulates gray lint that reduces airflow — remove the blower and clean the wheel if the filter is clean and limits are still tripping.

1. 4For 9-blink igniter failure: turn off the furnace power switch and close the gas shutoff. The Lennox SureLight® igniter is located in the burner assembly — it is a gray or white silicon nitride element mounted on a bracket with two wire leads. Remove the single mounting screw and unplug the connector. Do NOT touch the igniter element with bare hands — skin oils cause hot spots that crack silicon nitride. Handle only the bracket or wear gloves. Test resistance with a multimeter: a functional Lennox SureLight® igniter typically reads 40–90 ohms. OL (open circuit) means the element has cracked. Inspect visually for a hairline fracture across the element. Install replacement LB-100549G or 73K9701 — align the mounting holes, hand-tighten the screw, and reconnect the plug.
2. 5For 8-blink low flame signal: turn off power and gas. Locate the flame sensor rod in the burner assembly — a thin metal rod approximately 3 inches long, mounted with a ceramic white insulator, one wire connector. Remove the single mounting screw. Inspect the metal rod tip: a white or gray oxide coating is the signal thief. Buff the metal rod only (not the ceramic) with 120-grit emery cloth or fine steel wool until the metal is bright and clean — takes 30 seconds. Reinstall, restore power and gas, and run a heat cycle. The furnace should light and stay lit past the trial period. If 8 blinks returns after cleaning, check the ground strap from the furnace chassis to the gas valve — a poor chassis ground reduces flame rectification signal. Also check the wire connector at the control board (part 45K36) for corrosion.
3. 6Lockout reset procedure for Lennox SureLight® systems: set the thermostat to OFF. Flip the furnace power switch to OFF and wait 30 seconds. Restore the power switch, then set the thermostat to HEAT at least 5°F above current room temperature. The board clears the lockout on power cycle and begins a new startup sequence. If the furnace immediately re-enters the same blink code without completing a full ignition attempt, the root cause was not corrected — do not reset again until the fault is diagnosed. For iComfort or iHarmony communicating thermostat systems, fault codes also log to the thermostat's history menu and can be cleared there after correcting the underlying fault.

Repair vs Replace

Recommended Tools & Parts

• Lennox SureLight® Silicon Nitride Igniter (LB-100549G / 73K9701)

  OEM Lennox SureLight® replacement igniter for EL296V, SL280, EL180, ML180, SLP98V, SL297NV furnaces. Fixes 9-blink igniter failure code. Silicon nitride element — handle bracket only, not the element. Part numbers LB-100549G and 73K9701 are interchangeable on most series. H8908A fits some older Lennox model lines.

  $30–$75

  Buy on Amazon →

• Lennox Flame Sensor Rod

  Replacement flame sensor for Lennox gas furnaces. Fixes 8-blink low flame signal when cleaning fails to restore adequate rectification current. Match the mounting bracket style and wire length to your existing sensor.

  $15–$40

  Buy on Amazon →

• Lennox Draft Inducer Pressure Switch (62J04)

  Replacement single-port pressure switch for Lennox furnaces — fixes 3-blink pressure switch open and 2-blink pressure switch closed faults when hose is verified clear. Match the water column (WC) rating stamped on your existing switch.

  $20–$55

  Buy on Amazon →

• Lennox Control Board (45K36)

  Replacement SureLight® control board for Lennox furnaces. Resolves persistent blink codes that remain after all components have tested good and been replaced. Verify board compatibility with your specific model number before ordering.

  $80–$160

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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