Lennox Furnace Error Code 27 usually means the ignition trial failed or the control did not prove flame after the gas valve opened. The single most likely fix is cleaning and repositioning the flame sensor, then checking the igniter and burner grounding so the control board can reliably detect flame.
Turn the furnace off at the service switch or breaker, wait 15 to 30 minutes, then restore power and call for heat at the thermostat. This resets the control board and clears a temporary lockout. If the code returns on the next heat cycle, continue with the next steps.
The most common real fix is to clean the flame sensor with fine abrasive cloth and make sure it sits directly in the burner flame. A dirty sensor can stop the PCB from reading microamp flame current even when the burners light briefly. Also inspect the sensor wire and the burner assembly ground path.
If cleaning the sensor does not solve it, check the hot surface igniter, gas supply, burner crossover ports, and wiring harness between the igniter, gas valve, and control board. A weak igniter, poor ground, or unstable gas flow can cause ignition failure and trigger Error Code 27. If you smell gas, stop immediately and call a licensed HVAC technician.
On a Lennox furnace, Error Code 27 generally points to an ignition or flame-proving problem. The control board starts the inducer motor, verifies pressure switch operation, energizes the igniter, and opens the gas valve. If the burners do not light correctly, or if the flame sensor does not send a stable flame rectification signal back to the PCB, the furnace shuts down and stores the fault.
In practical terms, the furnace may try to start, ignite for a second or two, then turn the gas off and retry. After repeated failures, the control enters soft or hard lockout and displays the code. The issue can be as simple as oxidation on the flame sensor rod, but it can also involve low gas pressure, a cracked igniter, dirty burners, weak grounding, damaged wiring, or a failing control board.
Because Lennox Furnace Error Code 27 is tied to the ignition sequence, diagnosis should focus on components that create, deliver, and verify flame. That means checking the hot surface igniter, flame sensor, gas valve operation, burner carryover, pressure switch sequence, wiring harness connections, and the furnace control board. Skipping basic cleaning and inspection often leads to unnecessary part replacement.
The flame sensor is one of the most common reasons this code appears. If the rod is coated with carbon or oxidation, the furnace may ignite but the control board cannot read the flame signal, so it closes the gas valve within seconds.
A hot surface igniter can glow weakly, crack, or fail electrically and still look normal at a glance. If it does not reach proper ignition temperature, gas will not light consistently and the control will register an ignition failure.
Rust, dust, or debris in the burner assembly can disrupt flame carryover from one burner to the next. That can create delayed ignition, unstable flame, or partial burner lighting, all of which interfere with proper flame sensing.
If incoming gas pressure is low, the manual gas cock is partially closed, or the gas valve is not opening correctly, the burners may not ignite at all. This is a real safety issue and should be handled carefully because ignition faults can lead to repeated gas valve cycling.
Flame rectification depends on a clean electrical path through the burner and chassis back to the control board. Corroded terminals, a damaged wiring harness, or a failing PCB can interrupt that signal and trigger Error Code 27 even when flame is present.
Follow the steps below one at a time — many error codes can be fixed faster than they look.
Tools you may need: screwdriver, multimeter, flashlight
Safety warning: Disconnect power before opening any panels or touching internal components.
Start with a full reset. Turn off the furnace at the service switch or breaker, wait 15 to 30 minutes, then restore power and set the thermostat to call for heat. Watch the startup sequence through the burner sight glass if your unit has one: inducer starts, igniter glows, gas opens, burners light, blower starts.
Remove the access panel and inspect the flame sensor. It is usually a small metal rod mounted at the burner assembly with a single wire attached. Remove it carefully, clean the rod with fine emery cloth or a non-oily abrasive pad, wipe away residue, reinstall it, and make sure the rod sits directly in the flame path.
Check the hot surface igniter next. Look for white spotting, cracks, chips, or a broken element, and inspect the connector for heat damage. If you have a multimeter, measure continuity or resistance according to safe HVAC practice; an open igniter must be replaced, and a damaged connector or wiring harness should also be repaired.
Inspect the burners and burner crossover ports for rust flakes, dust, or soot buildup. Clean light surface debris carefully and make sure the burner rails are seated correctly. If the first burner lights but the flame does not carry smoothly across the burner rack, the furnace can lose flame proving and post Error Code 27.
Verify electrical connections and grounding. Check the flame sensor wire, igniter harness, gas valve terminals, and ground screws for looseness or corrosion. A weak chassis ground or oxidized burner mounting surface can reduce flame rectification current and make the control board think no flame is present.
Observe whether the igniter glows and whether gas is being delivered when the ignition trial begins. If the igniter glows but there is no flame, confirm the gas shutoff is fully open and that other gas appliances in the home are operating normally. Do not attempt to adjust manifold pressure or disassemble the gas valve unless you are qualified to do so.
If the furnace still fails after sensor cleaning, igniter inspection, burner cleaning, and wiring checks, the problem may involve the gas valve, pressure switch sequence, or control board. At this point, stop and call a professional with combustion test equipment and a manometer. Professional diagnosis is especially important if the furnace short-cycles, smells of gas, or repeatedly locks out.
Yes, you can try a basic reset by shutting off power to the furnace for 15 to 30 minutes and then restoring power. That may clear a temporary lockout, but if Lennox Furnace Error Code 27 comes back, the underlying ignition or flame-sensing problem still needs to be fixed.
It can be. Most cases involve the furnace safely shutting itself down because it did not detect proper ignition, but repeated gas valve cycling and failed ignition attempts should not be ignored. If you smell gas, hear delayed ignition, or see rolling flames, turn the furnace off and call a professional immediately.
The cost depends on the failed part and labor rates in your area. Cleaning a flame sensor is inexpensive and often a DIY maintenance task, while replacing an igniter, gas valve, or control board costs more. Professional diagnosis is usually worth it if simple cleaning and inspection do not solve the problem.
That symptom strongly points to a flame-proving issue. The burners ignite, but the flame sensor, grounding path, or control board does not confirm the flame signal, so the PCB closes the gas valve as a safety measure. Cleaning the sensor is the first thing to try.
Usually not directly. A dirty air filter is more likely to cause overheating and limit switch faults than an ignition code. However, poor maintenance often means the burner compartment, flame sensor, and venting may also be dirty, so it is still smart to inspect and replace the filter during troubleshooting.
Call for service if the igniter does not glow, the furnace smells like gas, the burners ignite violently, the code returns after cleaning the flame sensor, or you suspect a gas valve or control board issue. Anything involving live gas pressure testing or combustion analysis should be handled by a licensed HVAC technician.
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