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Furnace No Flame: Ignition and Sensor Failure Fix

Quick Answer

If your furnace ignites but shuts off after a few seconds, the flame sensor is likely dirty or oxidized. Cleaning the metal rod with a light abrasive usually restores the signal and stops the unit from locking out.

Most of the time when I show up for this call, the furnace is actually working fine. It's the safety circuit that's blind to it. The control board needs a tiny microamp signal from the flame sensor to keep the gas valve open, and when carbon builds up on that rod, the signal disappears. Ignore this and you'll be without heat on the coldest night of the year, usually around 2am.

GenericFurnaceSeverity: highDifficulty: intermediate88% DIY Success
Time to Fix
20–60 min
Difficulty
intermediate
Parts Cost
—
Tools Needed
Phillips #2 screwdriver, 1/4 inch nut driver

What Does the NO-FLAME-DETECTED Code Mean?

Nine times out of ten this is just a dirty flame sensor. The furnace tries to light three times, doesn't get confirmation that there's an actual flame, and locks out for several hours. I always check the sensor first because it costs nothing to clean and it's the most common failure point I see in modern high efficiency systems. Seriously, I cleaned three of these last week alone.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Dirty flame sensor65%
Failed igniter20%
Gas supply issue10%
Control board failure5%

Symptoms You May Notice

  • The furnace kicks on, you hear the inducer spin up and the igniter energize, then after about 5 seconds everything just stops. Then it tries again. Three attempts total before the whole unit gives up and locks out.
  • Blower fan runs full blast but the air coming out of the vents is room temperature or colder, because the burners never actually stayed lit long enough to heat the exchanger.
  • You hear a soft whump or a click as it tries to light, followed by silence, then the whole sequence starts over from scratch two more times.
  • The small LED on the control board is flashing a fault code that repeats in a pattern, usually 3 or 4 blinks with a pause, over and over.
  • No orange glow visible through the viewport window at all during startup, just the inducer fan running and then shutting down.

Can you reset a Generic furnace to clear the NO-FLAME-DETECTED code?

Flip the power switch on the side of the furnace to off and wait a full 60 seconds. Some boards won't clear if you rush it, so actually wait. Flip it back on, go to your thermostat, and bump the setpoint up 5 degrees above current room temp. The furnace will attempt a fresh ignition sequence. Don't reset it more than twice in a row without figuring out the underlying problem first.

Tools Required for Diagnosis

Phillips #2 screwdriver1/4 inch nut driver5/16 inch nut driverScotch-Brite pad or fine steel wool (not coarse sandpaper)Digital multimeter with microamp DC settingFlashlight or headlampSafety glasses

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

ComponentComponent Under Test
Expected Range1.5–10 microamps DC
ConditionIf Open (OL) or infinite, replace component.

Frequently Asked Questions

Can I use a dollar bill to clean a flame sensor?
Yeah it works in a pinch. I've actually done it myself when I forgot my kit and the homeowner needed heat that night. A crisp bill has just enough abrasive texture to knock off light oxidation. But honestly it's a short-term fix. Get a Scotch-Brite pad or some fine steel wool if you want it to last more than a few weeks. The dollar bill trick is strictly for 'I need heat tonight and the hardware store is closed' situations.
Why does my furnace blow cold air then shut off?
That's the safety sequence doing exactly what it's supposed to. The furnace tried to light, couldn't confirm a flame signal, and cut the gas. But the blower still runs for a bit to clear any unburned gas and cool things down. It's not dangerous, it's just broken. Usually means it failed all three ignition attempts and it's now in lockout. The fix is almost always a dirty flame sensor. Clean it first before you start replacing parts.
How do I know if my igniter is bad versus the sensor?
Watch the startup through the viewport. If the igniter never glows orange or glows really dim and weak, that's your igniter. If it glows nice and bright but the flame lights and dies within 5 seconds, that's the sensor. I've seen both fail at the same time on older units, so if cleaning the sensor doesn't solve it, test your igniter with a multimeter. Most hot surface igniters should read somewhere between 40 and 200 ohms depending on the type. Open circuit means it's toast.
What does three blinks on the furnace control board mean?
Three blinks on a lot of standard boards means pressure switch fault or ignition failure, but here's the thing: every manufacturer uses different flash codes. There's always a legend on the inside of the furnace door or printed right on the control board itself. Takes 10 seconds to check. Don't guess on this one because three blinks can mean something totally different depending on who made your board.
Is it safe to keep resetting the furnace?
Don't do it more than twice without fixing the underlying problem. Every failed ignition attempt means the gas valve opened briefly without a confirmed flame. Most of that gas clears out, but not all of it. If the igniter suddenly fires on a later attempt with residual gas sitting in the cabinet, you can get a small boom that cracks the heat exchanger or trips the rollout switch. Two resets max, then stop and diagnose what's actually wrong before you try again.
How often should I clean the flame sensor?
Honestly, once a year as part of your annual tune-up is plenty for most homes. Some units in dusty environments or ones that run constantly all winter might need it twice a year. I usually do it every fall before the cold hits. Takes about 10 minutes and it prevents the 2am no-heat call. If you're cleaning it more than twice a year, something else is wrong, either the burner combustion is dirty or there's a gas pressure issue worth investigating.
RP

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 17, 2026