A few times in the past week I will wake up to my furnace turning on, hearing a click when it tries to ignite, then turn off. This will happen for a couple minutes before the furnace quits trying.

However, after a certain period of time, it will turn on again with no issues.

What should I do so it doesn't happen ever?

  • What type of ignitor does it have (constant pilot, glow, spark, or something else)? What is the make and model? Is it a high efficiency model?
    – Tester101
    Nov 28, 2013 at 13:19
  • Spade connectors on furnace control boards can oxidize over a few decades to the point where proper electrical contact is lost. Depowering the furnace, unplugging, cleaning and replugging all the control connectors, then repowering fixed my flaky furnace. Nov 24, 2016 at 3:22

9 Answers 9


Furnaces are expensive, complex pieces of equipment. If you don't have the proper tools and/or knowledge, it's often best to let the professionals handle maintenance and repair.

Gas Furnace Ignition Sequence:

  1. Thermostat calls for heat.
  2. Pilot gas valve opens.
  3. Ignition control starts (spark or glow).
  4. Pilot gas ignites.
  5. Flame sensor detects pilot flame.
  6. Main gas valve opens.
  7. Main burners ignite.
  8. Blower timer starts.
  9. Flame sensor detects main burner flame.
  10. Blower starts.

High Efficiency Gas Furnace Ignition Sequence:

  1. Thermostat calls for heat.
  2. Draft inducer starts.
  3. Vacuum switch detects negative pressure.
  4. Pilot valve opens.
  5. Ignition control starts (spark or glow).
  6. Pilot gas ignites.
  7. Flame sensor detects pilot flame.
  8. Main gas valve opens.
  9. Main burners ignite.
  10. Blower timer starts.
  11. Flame sensor detects main burner flame.
  12. Blower starts.

Ignition Problems:

Vacuum Switch Not Closing

In a high efficiency furnace, if the vacuum switch does not close after a certain timeout. The furnace will shut down, then possibly retry a few times depending on model. If after a given number of tries the switch still does not close, the furnace will enter lock out. Once in lock out, the furnace has to be manually reset (depending on model).

The vacuum switch will look something like this...

Vacuum switch

And will have a rubber tube connecting it to the draft inducer. This part cannot be repaired, and must be replaced if it's faulty.

Pilot Not Igniting

If the pilot does not light, you'll first want to check to make sure there is something trying to ignite it. Typically a spark or glow ignitor is used, so first you'll want to determine which is being used.

Glow Ignitor

A glow ignitor looks similar to this, though may not be visible without further disassembly.

Glow ignitor

It works by passing a current through it, causing it to heat up and glow. It should heat up enough to ignite the pilot gas. If it doesn't heat up, it should be replaced.

Spark Ignitor

A spark ignitor looks like this.

Spark Ignitor

Take notice of the thick, often orange or red wire that is typically used to connect the ignitor to the ignition control module. This device works by generating an electrical arc between the two probes, causing the pilot gas to ignite. If there is no spark, you'll have to replace the ignition control module and/or the ignitor itself. If the ignitor is not igniting the gas, but there is a spark. You can try cleaning the electrodes with fine steel wool, to remove any carbon buildup.

Pilot Not Proving

For a pilot to "prove", it simply means that the flame sensor has sensed the pilot flame. If there is no pilot or the sensor doesn't detect it, the furnace will often purge the system and then try again. Furnaces will often try a set number of times, before entering lock out.

If you can see the pilot flame, but the sensor is not detecting it. Try gently cleaning the sensor with fine steel wool, to remove any built up carbon. Also make sure the sensor is in the proper location with respect to the flame (Check the owners manual for proper placement), and adjust as necessary. If that doesn't work, replace the sensor.

Main Burner Not Proving

If the main burner ignites, but the furnace shuts down before the blower starts. You'll want to check the main flame (or rollout, high limit, etc) sensor(s). If your furnace has error code indicators, check those and compare to the owners manual for translation. In this case, you'll want to test and/or replace each sensor and/or contact an HVAC technician

  • 1
    Out of all the great advice in this answer, this turned out to be the correct solution "If you can see the pilot flame, but the sensor is not detecting it. Try gently cleaning the sensor with fine steel wool, to remove any built up carbon."
    – Jason
    Apr 9, 2014 at 11:41

I have a similar problem, also. This is how I repaired my furnace: when it tries to come on to soon (like less then 10 minutes) it won't open the gas valve. Try to do this three times. When you hear a "click" sound tap the gas valve with a plastic handle of a nut driver for instance. The tapping should open the valve. If you can verify that the furnace stay's on for about 15 minutes before it turns off it should be fine. I believe that the solenoid at the valve may be the problem. It may be getting stuck and the tapping jars it loose. Hope this helps.


My furnace would ignite the pilot and energize the induction motor but the main burner valve would not open. At first I suspected a bad coil on the vale selonoid but continuity check indicated it was good. Then traced around with my meter and got to the pressure switch. It was not switching when the induction fan ran. I removed the tube from the furnace and could make it switch by gently applying a vaccum with my mouth. I ended up dismantling the induction intake plenum an found that over the years condensation from the airconditioner had dissolved some of the zinc plating and carried it down to the port of for the pressure switch tube. There it would evaporate and eventually built up enough of a zinc deposit to completely plug the port. Using a small drill bit turned by hand I cleared the port and reassembled the furnace. That was it, worked the first time and every time. Quite the learning experience!.


I did all the above checks on my gas furnace and none solved the issue.

Then I blew compressed air through each of the gas venturis (the hourglass shaped tubes that mix the natural gas with the air - flames come out the back). It turns out that over the years that the furnace was in service, the dust build-up in the venturis was enough to affect the gas flow and ignition across the venturis.

Of course if you are going to try this fix, turn the gas and power off first etc.


I had the same problem on a 25 year old furnace. I thought the exhaust was stopped up again, But it was just the vacuum switch causing it. It had dust build up in it. All I had to do was blow into the line connected to the sensor, and it was fine.


After cleaning the flame sensor, checking ground wire, checking exhaust for cracks or blockage and the board connection. We found that the hanger on the exhaust pipe was hung poorly and causing pipe to droop down which made it hold condensation that ran back into unit which shut the furnace down.

  • Hello, and welcome to Stack Exchange. Your answer would be better if it were more clearly an answer to the original posted question. Perhaps an initial sentence like "We had a similar situation in our house" would help. Jan 18, 2016 at 2:23

Mine would open the main valve and good flame and valve would close immediately but kept trying (a buzzing sound and hearing the repeat attempts took me to the basement). I thought a flame sensor but as I had just replaced a loud inducer motor, I pulled the vacuum hose off and applied vacuum and got water. Drained hose, made the vac. switch activate a couple times and no buzz and normal operating. Need chimney work...Guess a "T" trap for now.


Had one furnace that would get all the way through to lighting the main burners with good flame and then start blower motor. After 4 or 5 seconds the flame would shut off and after another 15-20 seconds (with blower still on) the flame sequence would start and light again. This would repeat two or three times. Turns out the flame would shut off on heat exchanger high temp limit, blower would cool down heat exchanger, and high limit switch, and flame sequence would start again. The rain cap on the flue stack above the roof had slid down and was choking off the flue exhaust flow over heating the heat exchanger. Fix was to install sheet metal screws fixing the rain cap in the proper position. Operation normal after that.


My problem was close to the same everything worked as it should but the flame would only stay lit for 7 seconds. I followed the trouble shooting tree in the install guide. It said "flame lights but won't stay lit-check flame sensor l.e.d. if flashing clean or replace the sensor." I cleaned it, with out removing it, with fine steel wool. The l.e.d. light stopped blinking and stayed on with the flame and the furnace heats until the thermostat is satisfied. Hope this helps most install guides are in a pouch on the furnace.

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