Lennox furnace error 270 is an ignition lockout after multiple failed attempts. The furnace tried to light the burners and failed each time. Start by cleaning the flame sensor rod with 0000 steel wool: the thin oxide coating on the sensor surface blocks the microamp signal the board uses to confirm flame.
When I see a 270 on a Lennox SL280V, it usually means the furnace tried to light three times and gave up for safety. It's incredibly common after a long summer of sitting idle because dust and oxidation build up on the internals. Sounds scary, but most homeowners can get their heat back on by focusing on the ignition components rather than assuming it's the expensive control board.
Multimeter with DC microamps and resistance modes, Phillips #2 screwdriver
What Does the 270 Code Mean?
In the field, I treat this as a communication breakdown. The gas might actually be lighting, but if the furnace can't prove it, the system shuts down to prevent a gas buildup. Before you call a pro, we need to figure out if the burners are actually catching or if the ignitor is just glowing into empty air.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Hot surface ignitor cracked or thermally degraded40%
Flame sensor rod coated with oxide film24%
Gas valve not opening on command22%
Gas supply pressure low or manual shutoff partially closed14%
Symptoms You May Notice
Code 270 flashing on the control board LED or showing on the iComfort thermostat display
You can see the ignitor glow orange through the sight glass but the burners never kick on, or they light for maybe 5 seconds and immediately cut out
Furnace was catching maybe 1 out of 3 start attempts for a few days before it went into full lockout
You reset it, heat works for one cycle, then back to 270 within the hour
Faint gas smell near the furnace right after a failed start attempt, which actually means gas IS reaching the burners and ignition is the real problem
Can you reset a Lennox furnace to clear the 270 code?
To clear the lockout, flip your thermostat to off or cycle the furnace power switch for 30 seconds. On newer Lennox Communicating systems, you might need to clear the alert at the iComfort thermostat. Just remember that resetting is a temporary fix. If the underlying carbon buildup or ignitor crack is still there, that 270 code will be back within an hour.
Tools Required for Diagnosis
Multimeter with DC microamps and resistance modesPhillips #2 screwdriver1/4-inch magnetic nut driver0000 steel wool or fine emery clothFlashlight or headlampWork gloves
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
ComponentComponent Under Test
Expected Range40–90 ohms
ConditionIf Open (OL) or infinite, replace component.
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
Part Name
OEM Number
Estimated Price
Hot Surface IgnitorH0909082A · $25–$50
H0909082A
$25 – $50
Flame Sensor10L61 · $10–$20
10L61
$10 – $20
Frequently Asked Questions
How do I clean the flame sensor on a Lennox SL280V or EL296V furnace?
Shut off the power and remove the single 1/4-inch screw holding the sensor in place. Pull it out and use a small piece of 0000 steel wool to polish the metal rod. You aren't trying to grind it down, just knock off the invisible oxidation layer. Pro tip: if you don't have steel wool, a clean dollar bill actually has enough abrasive texture to clean a sensor in a pinch. Avoid coarse sandpaper though, it creates scratches where carbon can hide and you'll be dealing with another 270 code next month.
What causes Lennox furnace code 270 to keep coming back?
If you've cleaned the sensor and the code keeps returning, check your furnace grounding. Lennox boards are sensitive to electrical noise and a loose ground wire makes the flame signal erratic. I've also seen it happen repeatedly when the burner ports are dirty right where the ignitor sits, which prevents the gas from reaching the heat source fast enough during the short ignition window. Had one last winter where the homeowner had cleaned the sensor three times. Turned out the green ground screw on the burner box was barely finger-tight. Twenty seconds with a screwdriver and it never came back.
Can I test a Lennox furnace flame sensor without a multimeter?
You can do a visual timed test. Watch the furnace start. If the flame lights but shuts off after exactly 5 to 7 seconds, the board isn't receiving the flame signal and that 'start and stall' pattern is the classic dirty sensor symptom. If the gas never lights at all, your problem is the ignitor or gas valve, not the sensor. The 270 code is the board's way of saying it ran the full sequence and gave up to protect your home from unburned gas buildup.
How much does it cost to fix a Lennox 270 error code?
Depends entirely on what's causing it. Cleaning the flame sensor yourself is basically free, 15 minutes and a piece of steel wool. A new hot surface ignitor runs $25 to $60 for the part, or $150 to $250 if you're paying a tech with labor. Gas valve replacement gets expensive, usually $300 to $500 installed. Control board is the worst case, often $400 to $700 depending on the model. Honestly, in my experience about 70% of 270 codes are just a dirty flame sensor or a lazy ignitor. Start there before anyone tells you the board needs replacing.
Is it safe to keep resetting the furnace when it shows a 270 code?
You can reset it once or twice to try and get some heat going short-term, but don't make it a habit. Every failed ignition attempt means the gas valve opens, dumps gas into the heat exchanger for a few seconds, and nothing lights it. That gas dissipates, it's not building up dangerously in your house in normal circumstances, but it's still not ideal. If you're resetting it more than twice in a row, stop and actually diagnose the problem. And if you smell gas near the furnace that isn't clearing within a minute or two of a failed start, shut everything down and call your gas utility.