Lennox furnaces use a built-in diagnostic system that flashes a LED on the control board or displays digital fault codes on a communicating thermostat. These codes tell you exactly what triggered the safety shutdown. This guide covers both systems so you can decode the message before calling a technician.
How to Read Lennox Flash Codes (Older Models)
On Lennox Merit, Elite, and older Signature Series furnaces (G61MP, SL280, EL296, ML196E), the control board has a single LED visible through a small window on the furnace door. Count the number of rapid blinks, then wait for the pause, then count again. That number is your fault code.
Lennox Flash Code Reference Chart
| Blink Count | Fault | Most Common Cause | DIY or Call? |
|---|---|---|---|
| 1 blink | Normal operation - no fault stored | System is running correctly | No action needed |
| 2 blinks | Pressure switch stuck open | Blocked exhaust vent, kinked pressure hose, or failed inducer motor | Check vent for ice/obstruction; call if clear |
| 3 blinks | Pressure switch stuck closed | Pressure switch has failed closed; control board issue | Call technician |
| 4 blinks | High-limit or secondary-limit open | Clogged air filter, blocked return air, or failed blower motor | Replace filter first; call if issue persists |
| 5 blinks | Flame sensed with no call for heat | Gas valve leaking or stuck open | Call immediately - gas safety issue |
| 6 blinks | Rollout switch open | Cracked heat exchanger, blocked flue, or improper combustion | Call immediately - CO risk |
| 7 blinks | Four consecutive ignition failures | Dirty flame sensor, failed ignitor, low gas pressure, or gas valve issue | Clean flame sensor first; call if no fix |
| 8 blinks | Inducer motor running before startup signal | Control board fault or stuck inducer relay | Call technician |
| 9 blinks | Ignitor not proven (no glow detected) | Failed hot surface ignitor or loose wiring to ignitor | Call technician (ignitor replacement) |
| Rapid continuous flash | Reverse polarity or low flame signal | Incorrect wiring or extremely dirty flame sensor | Clean flame sensor; check wiring polarity |
Lennox iComfort and Communicating System Error Codes
Newer Lennox Signature Collection and Dave Lennox Signature (SLP98V, ML296V, EL296V) paired with an iComfort S30 or E30 thermostat display digital fault codes. These are more precise than flash codes.
| Code | Fault Description | Likely Cause |
|---|---|---|
| E223 | Pressure switch open | Exhaust vent blocked, pressure hose disconnected, or inducer fault |
| E225 | Pressure switch stuck closed | Failed pressure switch, condensate drain blocked |
| E261 | High-limit event | Dirty filter, blocked supply registers, or blower motor failure |
| E233 | Rollout switch open | Heat exchanger crack or blocked flue - call immediately |
| E526 | Four ignition failures | Dirty flame sensor, failed ignitor, or gas supply issue |
| E269 | Inducer proving fault | Inducer motor not reaching speed; motor or control board fault |
| E301 | Communication loss with furnace | Wiring fault between thermostat and furnace control board |
| E503 | Blower motor fault | Variable-speed ECM blower motor failure or wiring issue |
The Two Most Common Lennox Fault Codes in Ontario
4 Blinks / E261: High Limit Event (Overheating)
This is the most common Lennox fault call we receive across the GTA. The furnace overheats and the high-limit safety switch cuts power to the burners. The furnace will often restart once it cools, then shut down again - leading to an on-again-off-again heating cycle.
- Step 1: Turn off the furnace and check the air filter. A filter clogged with dust is the cause in roughly 60% of high-limit calls.
- Step 2: Walk through your home and confirm all supply registers and return air vents are open and unblocked by furniture.
- Step 3: If the filter is clean and registers are clear, check that the blower fan runs continuously. If it runs briefly then stops before the burners shut down, the blower motor may be failing.
- Step 4: If the issue continues after changing the filter, call EcoFrost. Repeated high-limit trips can crack the heat exchanger.
7 Blinks / E526: Ignition Failure
The second most common fault in Ontario, especially after a season of heavy use. The furnace tries to ignite four times, fails, and locks out for safety. It will usually try again after 60 minutes or when you cycle the thermostat.
- Most common cause: a dirty flame sensor. The flame sensor is a thin metal rod that confirms gas is actually burning. A coating of oxidation or debris prevents it from detecting the flame.
- Second most common: a failed hot surface ignitor. The ignitor glows orange before ignition. If it does not glow, the ignitor has likely burned out.
- Less common: low gas pressure due to a partially closed shut-off valve or supply issue.
- DIY option: Cleaning the flame sensor with very fine steel wool or emery cloth can resolve the fault temporarily, but full verification requires a technician.
Flash Code 6: The One You Should Never Ignore
Six blinks means the rollout switch has opened. The rollout switch trips when flames are rolling out of the heat exchanger opening instead of staying inside the combustion chamber. This almost always indicates a cracked heat exchanger, a blocked flue pipe, or improper combustion.
How to Reset Your Lennox Furnace After a Fault Code
For most fault codes (2, 3, 4, 7, 8, 9), you can attempt a manual reset after addressing the likely cause. Set the thermostat to OFF, wait 30 seconds, then set it back to HEAT with a setpoint above room temperature. The furnace will attempt a new startup sequence. If the same code returns immediately, the underlying problem has not been resolved and continuing to reset without a repair can damage components.
Ready to take the next step?
Book a Lennox Furnace DiagnosticWhen to Call a Technician Instead of Resetting
- Code 5 or 6 (or E233) - gas valve or rollout switch fault. Do not reset.
- Code 7 that returns within one heat cycle after resetting - ignitor or gas supply needs professional diagnosis.
- Code 4 that returns immediately after changing the filter - blower motor or heat exchanger issue.
- Any code accompanied by a burning smell, gas smell, or CO alarm activation.
- Flash codes on a furnace older than 15 years - repair cost vs. replacement value should be assessed.
?Frequently Asked Questions
How do I find the LED on my Lennox furnace?
My Lennox furnace blinks 4 times but the filter is clean - what now?
Can I reset a Lennox furnace lockout myself?
What does a solid red LED mean on a Lennox furnace?
How much does Lennox furnace repair cost in the GTA?
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