TL;DR
Lennox furnaces don’t all speak the same diagnostic language, and neither format works the way a lot of fault-code lists describe. Look at your control board first:
- Two LEDs (usually one red, one green)? Faults are read from the combination of both LEDs flashing together, alternating, or one steady while the other flashes. Not a single number. Table below.
- A small digital display showing letters and numbers (like E110)? That’s an iComfort-enabled/communicating control. It shows a 3-digit code directly, no counting required. Table below.
Always confirm against the sticker inside your furnace’s door panel. Lennox has used multiple control-board generations, and yours is the authority for your exact unit.
Last reviewed: August 17, 2026.
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Lennox is a dealer-only brand. Unlike Goodman or Carrier, you can’t buy Lennox parts at a big-box store or general HVAC supply house. If a component needs replacing, you’ll typically go through a Lennox dealer or authorized service company.
How to read a Lennox furnace fault code (correctly)
- Open the lower door panel (slides off or unclips) to see the control board.
- Look at the board: do you see two LEDs, or a small digit/letter display? This determines which table below applies to you.
- If two LEDs: watch both at once for at least two full cycles. The fault is defined by the pair: which one is on, off, or flashing, and at what rate (slow ≈ 1 flash/second, fast ≈ 3 flashes/second).
- If a digit display: read the 3-character code directly (e.g., “E110”). No counting needed.
- Cross-check against the sticker on the inside of your door panel. Lennox control-board generations differ, and the sticker is written for your specific unit.
A note on why this matters: guides that describe Lennox codes as “4 blinks = code 4” or “count to 23” are describing how other manufacturers’ single-LED boards work, not what Lennox’s own installation manuals document. We could not verify a single-LED, count-the-blinks system on any Lennox integrated control we checked against official documentation.
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System 1: Two-LED diagnostic codes (conventional, non-communicating controls)
Used on non-communicating single- and two-stage Lennox integrated furnace controls. Slow flash ≈ 1 Hz; fast flash ≈ 3 Hz.
| LED #1 (red) | LED #2 (green) | What it means | DIY or call a pro |
|---|---|---|---|
| Simultaneous slow flash | Simultaneous slow flash | Normal operation: idle, cooling, or continuous fan | Normal |
| Simultaneous fast flash | Simultaneous fast flash | Normal operation: heating demand active | Normal |
| Slow flash | Steady ON | Primary or secondary limit switch open (must close within 3 minutes or the unit enters a 1-hour Watchguard lockout) | DIY: check filter first |
| OFF | Slow flash | Pressure switch open: blocked inlet/exhaust vent, or pressure switch closed before the inducer started | DIY check |
| Alternating slow flash | Alternating slow flash | Watchguard 1-hour lockout. Burners failed to ignite or lost flame 5 times in one heating demand | Call a pro if it recurs |
| Slow flash | OFF | Flame sensed without the gas valve energized | Call a pro |
| Steady ON | Slow flash | Rollout switch open, or the control’s 12-pin connector is seated incorrectly | Stop furnace; call a pro |
| Steady ON | Steady ON, or ON/OFF, or OFF/ON | Circuit board failure or control wired incorrectly | Call a pro |
| Fast flash | Slow flash | Main power polarity reversed (line/neutral swapped) | Electrician or pro |
| Slow flash | Fast flash | Low flame signal: flame sense current below 1.5 microamps | DIY: clean flame sensor first |
| Alternating fast flash | Alternating fast flash | Improper main ground, or line voltage below 90V | Call a pro |
Source: Lennox ML180UH Installation & Operation Manual, p.39. This exact table is documented for the ML180UH; other conventional Lennox furnace lines use the same two-LED logic but individual fault assignments can shift between board revisions. Confirm against your door sticker.
System 2: Alphanumeric E-codes (iComfort-enabled / communicating controls)
Used on communicating-capable Lennox furnaces (iComfort-compatible models like SLP98, SLP99, and communicating-configured SL280/EL296 units). The control’s 7-segment display shows the code directly, with no flash-counting involved.
| Code | Fault | What it means |
|---|---|---|
| E105 | Communication problem | No other devices detected on the RS bus; usually wiring or electrical noise |
| E110 | Low line voltage | Incoming power below the furnace’s rated voltage |
| E111 | Line voltage polarity reversed | Line and neutral wiring swapped |
| E112 | Ground not detected | Furnace isn’t properly earth-grounded; shuts down until corrected |
| E113 | High line voltage | Incoming power above the furnace’s rated voltage |
| E115 | Low 24V | Control transformer voltage below 18V; often an overloaded transformer |
| E124 | Thermostat signal missing | No communication from the thermostat for more than 3 minutes |
| E125 | Control hardware failure | Internal self-check failure; cycle power, replace board if it persists |
| E126 | Internal communication error | Control-board internal fault; cycle power, replace if persistent |
Source: Lennox SL280UHV Installation Instructions, pp.60–61; consistent with the SLP98UHV integrated control diagnostic codes page. This is a representative subset. The full alert-code set runs well past 100 entries and covers communicating-system-specific faults (zoning, outdoor unit, humidifier) not relevant to a standalone furnace.
On a connected iComfort thermostat, active alerts also surface on the thermostat’s own screen with a plain-language description, in addition to the code shown on the furnace control. Check the thermostat’s alerts or status screen rather than relying on the furnace LED alone.
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What to check first, by fault category
Limit switch open (two-LED: slow/ON)
Start with the filter. A clogged filter restricting airflow is the single most common cause of a limit-switch trip. Replace it, cut power for 30 seconds, restore power, and call for heat; the limit resets automatically once the heat exchanger cools. If it returns immediately: check that supply registers are open and nothing blocks the return air grille. Some older Lennox limit switches are manual-reset only (a tech has to reset them), so if a power cycle doesn’t restore heat, that may be why. Recurring trips on a furnace over 15 years old warrant a heat-exchanger inspection before winter.
Pressure switch open (two-LED: OFF/slow)
On 90%+ AFUE condensing furnaces, this is almost always a condensate drain issue: find the clear/white drain tubing, locate the trap, and clear it with a wet-dry vac on the outlet or a water flush. On 80% AFUE units, check the flue pipe for disconnected joints, blockage, or ice at the exterior cap. If the inducer motor isn’t audibly running, the motor or its capacitor may have failed. That’s a pro call.
Watchguard lockout (two-LED: alternating slow/slow)
The control locked itself out after 5 failed ignition or flame-loss attempts in one call for heat. Don’t keep power-cycling it to force a restart — that masks a real ignition problem. Check the flame sensor first (below); if cleaning it doesn’t resolve repeat lockouts, call a pro.
Flame sensed without gas valve energized (two-LED: slow/OFF)
The control is seeing a flame signal when it shouldn’t. This is a safety-relevant fault: it can indicate a stuck-open gas valve or a control fault. Don’t attempt to force a reset repeatedly. Call a Lennox dealer or licensed HVAC technician.
Rollout switch open (two-LED: ON/slow)
Flames extended outside the normal burner envelope. This can mean a blocked flue, dirty burners causing delayed ignition, or a cracked heat exchanger.
Safety box: stop using the furnace and turn off the gas supply valve. Do not attempt repeated resets. Call a Lennox dealer or licensed HVAC technician to inspect before restarting. Flame rollout, gas-valve faults, and heat-exchanger concerns are outside DIY scope on any furnace, Lennox included.
Low flame signal (two-LED: slow/fast; also covered by E-codes in the flame-sense family on communicating units)
Usually a dirty flame sensor.
- Turn off furnace power.
- Locate the flame sensor: a metal rod with a ceramic mount inside the burner box, single wire.
- Disconnect the wire, remove the mounting screw.
- Clean the rod with 0000-grade steel wool or fine emery cloth (not the ceramic mount).
- Reinstall, restore power, test.
If cleaning doesn’t resolve it, the sensor itself or the flame-sense circuit may need replacement. See our flame sensor guide.
Low/high line voltage, reversed polarity, ground faults (E110/E111/E112/E113, or fast-flash LED combos)
These point to the building’s electrical supply, not the furnace itself. Reversed polarity and grounding issues need a licensed electrician; voltage out of range needs the utility or panel checked. Don’t keep resetting the furnace against an unresolved electrical fault.
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Lennox communicating systems (iComfort)
If your furnace is connected to an iComfort thermostat, active alerts and fault history show up on the thermostat’s own screen in addition to the furnace control’s E-code display. Check there first; it’s more informative than reading the board. Lennox dealers can also pull fault history remotely if the system is connected.
Parts availability note
Lennox sells through an authorized dealer network:
- OEM parts (control boards, gas valves) are typically ordered through a Lennox dealer, not a general HVAC supply house.
- Common wear parts (igniters, flame sensors, capacitors) that fit Lennox units are often available from general HVAC suppliers using cross-reference part numbers.
- For a specific Lennox control board or gas valve, expect to go through a dealer.
When to call a Lennox dealer or HVAC technician
- Flame sensed without a call for heat, or a rollout fault: immediately. Stop the furnace, shut the gas valve.
- Watchguard lockout that recurs after cleaning the flame sensor and checking the filter.
- A limit-switch trip that returns after a filter change and clear return-air path.
- Any electrical-supply fault (voltage, polarity, ground). That’s an electrician or utility issue, not a furnace repair.
- Need for OEM parts specific to your control board or gas valve.
FAQ
Why does this table look different from other Lennox fault-code lists I’ve seen? Many guides describe Lennox codes as a single LED counting off a number (like “4 blinks = code 4”). We checked that claim against Lennox’s own installation manuals for both a conventional two-stage furnace and a communicating/iComfort-capable model and couldn’t verify a single-LED counted-blink system on either. What Lennox actually documents is two LEDs read in combination, or a 3-digit alphanumeric code on a display, never a single number from counting flashes.
Can I work on my Lennox furnace myself, or do I need a Lennox dealer? Most diagnostic work and common repairs (checking the filter, cleaning or replacing the flame sensor, replacing the igniter) are DIY-appropriate and don’t require a dealer. A Lennox dealer is typically needed for warranty work, iComfort/communicating-system faults, or a control board or gas valve replacement.
My furnace is 15+ years old. Are parts still available? Common wear parts (igniters, flame sensors, capacitors) for Lennox furnaces are widely available from general HVAC suppliers via cross-reference numbers, well past the point a model is discontinued. OEM-specific parts like a control board or gas valve are more likely to require a Lennox dealer, especially on older units. Ask before assuming a part is unavailable.
A limit-switch fault clears but keeps coming back. What does that mean? This is typically a worsening airflow restriction or a failing blower. The furnace overheats, the limit trips, it auto-resets after cool-down, and the cycle repeats. Common causes: a filter filling up faster than your change schedule accounts for, a dirty blower wheel, or a weakening blower motor capacitor. Have the blower compartment inspected if a fresh filter doesn’t fix it.
Related guides
- Furnace Not Heating: Causes, Diagnostic Order, and What to Do First: if no fault indicator is showing at all
- Furnace Flame Sensor: Symptoms, How to Clean It, and When to Replace: for low-flame-signal and Watchguard-lockout faults
- Furnace Hot-Surface Igniter: Symptoms, Testing, and Replacement: no glow through the sight glass during ignition
- Lennox vs. Carrier, Lennox vs. Rheem, Lennox vs. Trane, Lennox vs. Goodman: brand comparisons
