Troubleshooting a Coleman Forced-Hot-Air Furnace Limit Switch

Let's just get the disclaimer out of the way first: many homeowners may lack the tools, the experience and the ability to mess around with their heating system. I certainly understand that and agree: I leave most things to the pros.

On the other hand, if you wake up in a blizzard and find yourself without heat as I did this morning, there are a few things that you might want to try before you give up and call for repair.

Even if you can't fix the problem yourself, you may be able to determine at least some information about what is causing the malfunction, and that information can sometimes be very useful to the people who will ultimately come to fix it.

As I said, we had a nasty snowstorm last night. It wasn't a lot of snow, but what came down was wet, heavy and sticky. Of course the wind was howling too; high winds are what defines a storm as a blizzard.

We lost power several times overnight. I know that, because every time the power is restored, our bedside wireless telephone chirps "Press Menu for setup instructions." I heard that at least four times last night.

At 5:30, I hauled myself out of bed. The power had come on again, so I walked to the living room to turn up the thermostat. We keep the heat dialed way down overnight; it is programmed to turn back up at 6:00 AM, but I was up earlier as noted.

I turned it up and the furnace did kick in. I went to the kitchen and snagged a yogurt from the fridge. I had barely got to the drawer where we keep the spoons when the electricity went off again.

Just a few moments later, it kicked in again and shortly after that I heard the furnace start up. I sat down at my computer and started some work.

Perhaps a half hour later, the lights went out again. This time, they stayed out until just before my wife got up—possibly the chirping phone-setup message woke her up this time. We both sat down at our computers after eating a banana and a few nuts.

A few hours later, my wife noticed that the house was getting cold. I got up to look at the thermostat and saw that it should have been calling for heat, but the furnace was not responding. I shut the thermostat off and turned it back on, dialed down the temperature and dialed it back up, but got no response from the furnace. I could hear the relays click in the thermostat, so I suspected it was not the culprit and moved on to look at the furnace itself.

Coleman furnace. The covers are a pain to take off and put back.
Coleman furnace. The covers are a pain to take off and put back.

The covers on these units are a pain to take off and put back on. I have to do that when I change the filters and it is never fun, so my first attempt at fixing our lack of heat was to open the home circuit breaker cabinet, find the one marked "Furnace", and shut it off. I counted to ten, turned it back on, and was rewarded with the sound of the furnace kicking in.

That's good, I thought, and went back to my work. Unfortunately, it was not good: after the long air purge cycle, which is designed to blow out any lingering explosive gases that might have found their way into the ducts, the furnace did not fire.

Obviously I had a partially working furnace, but it did not want to fire.

Neither furnace manufacturers nor home owners like furnaces that blow up, catch on fire, or leak poisonous gases. That's why the units have various safeguards built in, like the air purge before firing off.

Many furnaces today have some sort of diagnostic indicator that can sometimes tell you what is wrong. For my furnace, it's a flashing green light and the cover of the control box helpfully tells you just what these lights mean.

The charge that follows is specific to MY furnace. Google your furnace model if you don't find a chart inside the unit.

Control box
Control box
  • One flash: Ignition lockout
  • Two flashes: Pressure switch stuck closed
  • Three flashes: Pressure switch stuck open
  • Four flashes: Limit circuit open
  • Five flashes: Gas valve stuck open
  • Six flashes: Wired wrong
  • Steady green: Working properly

But where is this flashing light? Well, you can see it if you step to your right and look at the side of the control box. It's not particularly easy to see, but you do not have to remove anything else to see it. The peephole is just above the blue wires in the picture below.

If you take off the cover of the control box, you can see the light quite plainly.

Diagnostic light with cover off
Diagnostic light with cover off

In a previous furnace failure, the light had been blinking three times, which means that the combustion air switch failed to close. It was a faulty unit. I did not attempt to replace it myself, but I did let the service company know what they'd need to bring. In this case, they had the proper switch on their truck anyway, so it wouldn't have mattered, but of course it could have.

In this case, the light was flashing four times. That means a limit switch was stuck open. I called the service company and told them.

When he arrived, he showed me how to follow the diagram to find the switches. If you look closely at the wiring diagram on the cover of the control box, you can see that an "upper limit switch" and a "lower limit switch" are referenced.

The diagram also shows the colors of the wires that go to these switches. The tech followed the colors and found that the lower switch was a small circular disk that had only the two wires coming into it. When we followed the gray and blue wires for the upper switch, we found something that had a button on it.

Buttons like that are usually reset buttons, so he pushed it.

Upper limit switch
Upper limit switch

I then cycled the furnace circuit breaker again and this time the furnace fired off after its purge.

That was a welcome sound! We let it run a bit, shut it off using the thermostat, waited a bit, and turned the thermostat up. The furnace worked again, so we were both satisfied that the issue was fixed.

The next time this happens, of course, I will try resetting that switch before I call for service.

While I had heating systems on my mind, I decided to change the batteries in my thermostat. It's easy to forget about these—they draw very little current, so the batteries can last for years. It's a good idea to replace them more often than that, though. As we use rechargeables, I just change them whenever something makes me think about the thermostat—that's often enough that I shouldn't ever have a problem.

Batteries in thermostat
Batteries in thermostat

Update: Summer 2013

Last winter we had frequent furnace problems. It usually wasn't the high limit switch: in fact, no lights would be flashing. I found that if I turned the thermostat to "Off" and then back to heat, it would work.

I bought a new thermostat, but that changed nothing. Thinking it might still be that limit switch, I ordered one of those.

That switch was backordered, and took so long to arrive that it was now summer. I decided I'd wait for winter again before replacing the switch, but then we had a spell of very hot weather. We don't usually use our air conditioner, but when the house hit 95 degrees, I turned it on.

But it didn't come on. When I checked outside, the big A/C unit was running, but the furnace fan never came on to blow the air. Fortunately, there is a switch on the thermostat to turn the fan on manually, so that's what I did, and we survived the heat wave.

Given that and the problems of the previous winter, I called in a service tech. He found the problem very quickly: a fuse.

No, not a blown fuse, but apparently one that would sometimes arc a bit.

Bad fuse in furnace
Bad fuse in furnace

You can see where it is in the left-hand photo above. A close up of the fuse (just an ordinary 3-amp automobile type fuse) points out the dark area where it had been arcing.

Replacing that fixed the problem instantly, and the technician felt it would likely also eliminate the heating problems too.

I'm going to pick up one of these to have on hand. This one cost me $88.00, which is worth it for the education, but it will be a whole lot less expensive next time!

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Comments 46 comments

LeAnn 5 years ago

Thank you! This is exactly what happened to us this morning. Thank goodness you have the same heater I do, it was very easy to figure out what to do.

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Pcunix 5 years ago from SE MA Author

I'm so happy for you. It cost me $225.00 to learn this!

bitty 5 years ago

thank you so very much, i am a single mother, live with my 2 kids, power went out at 1 am, snowing outside waiting for 9 inches, my furnace was lighting 4 times. i was about to call for repair !! I do appreciate this post, God bless your heart ...

Melissa 5 years ago

thank you so much for the info it was exactly my problem and after thinking I was going to freeze tonight.. you saved the day.. the pictures helped a lot.. the last picture is beside the blower if that helps anyone the little reset button.. Thank god for good people online to share info.

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Pcunix 5 years ago from SE MA Author

I'm happy to have saved you from a miserable night, but I can assure you that there were no gods involved :-)

Heather 5 years ago

Thank you soo much you saved me allot of money I couldent afford. Thank you for the pictures and the way you explained it!!!!

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Pcunix 5 years ago from SE MA Author

I'm so happy that at least a few people have had success here. Far too often people are afraid to even look for simple things like reset switches.

Cesar 4 years ago

Thanks for puttin this up, I was Gavin prob all night over this lil limit switch!

R-dawg 4 years ago

LIFESAVER!!!! This was exactly what was wrong with my furnace. Now....if only I had seen this 5 months ago....I would have had air conditioning as well over the summer lol.....thanks for posting the pics as well they helped me locate everything perfectly

Dorea 4 years ago

Thank you thank you thank you. You saved me so much.

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Pcunix 4 years ago from SE MA Author

I was just talking to someone yesterday who paid $150.00 for someone to push that switch..

Heather 4 years ago

Thank you so much for your help. I am a single mother with 5 kids and i was really worried this morning when my heat wasn't working. I did not have the funds to call someone. So thank you for all your help I got it going again.

CallTux 4 years ago

I had the same issue several years ago. Fortunately, it only cost me about $50 to learn about resetting the limit switch. This year, the limit switch started tripping every couple of hours, so I pulled it out, looked at it under a magnifying glass to find the part number, did a search and found a replacement. The exact same part number cost $25 from one store (with shipping) and $95 from another, so it pays to shop around.

By the way, if you have a problem with the furnace flame sensor, you can usually clean that with steel wool to remove burnt dust and any oxidation to make it as good as new.

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Pcunix 4 years ago from SE MA Author

Thanks, Calitux.

Can you believe that I got an email from someone complaining that I was giving out "dangerous" advice? Unbelievable.

Brian 4 years ago

Thank you for the post! Good pictures!

kate 4 years ago

thank you!

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Pcunix 4 years ago from SE MA Author

I'm so glad this has helped so many people.

Molly 4 years ago

Thank you so much you saved me

George 4 years ago

Great article! Thank you very much!If it was blinking 3 times again would you try and replace the switch yourself?

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Pcunix 4 years ago from SE MA Author

Well, *I* would but I wouldn't necessarily say that your average homeowner should.

Lennie 4 years ago

Thank you for this fantastic post.. Mine turned out to be the pressure switch $18.00. My green led didn't give any error codes.. I smacked the switch with my flash light and Ta Da, the thing started working so I replaced it.. Coleman has replaced all of the .20 pressure switches with .10.. purge time is a little shorter but Im a happy camper.

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Pcunix 4 years ago from SE MA Author

Smacking stuff does sometimes make it work - I'd be careful smacking stuff around a gas furnace though!

Octavio 4 years ago

Thanks for all the info in this section, had a similar problem I started by replacing the termostat with no possitive outcome I end up pushing the same buton on you pic and problem fix, thanks for the ilustrations.

denny 4 years ago

i have a colman furnace that seams to short cycle all the time and does not get hot enough to make the blower pump out heat. I had some warenty work done on it and it worked better for a while now back to the same thing. help please

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Pcunix 4 years ago from SE MA Author

I can't help you, sorry. All that I know is here on this page.

Jenn 4 years ago

Thanks so much for posting this. The power went out a couple of times yesterday and when we got up this morning it was very cold in the house. Two minutes after reading this the furnace is working again and things are warming up. I am very grateful that I don't have to bundle my kids up in their snow gear just to stay in the house!

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Pcunix 4 years ago from SE MA Author

That's great to hear! I'm glad you didn't need to do that, too.

It's surprising how common this is!

CerriThanks 4 years ago

Thanks so much for your input. Similar story happened to us. First the combustion switch, than about six months ago, now tonight the power in our neighborhood was out. We took our kids out to dinner and came hack to power, but no heat. We saw the light flashing 4 times and saw on the panel it was the limit switch, but we couldn't find it. Thanks to your post, we did, and we have heat tonight!!!! Thank you, thank you and thank you!

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Pcunix 4 years ago from SE MA Author

I'm so happy for you. I had to pay for someone to show me how to find it! But never again.

Brett Christensen 4 years ago

Thanks for the details on the limit switch. You saved me a service charge! Never would have saw that reset on the limit switch if I hadn't seen the detailed pictures.

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Pcunix 4 years ago from SE MA Author

Yes, you wouldn't expect to find it there. But you can follow the wires, too.

twalker 4 years ago

Thanks for putting this info out there. Saved us a bundle and a cold night! Big respect sir.

tim 4 years ago

hello and thank you mine isn't a green light but a red one but its doing everything that mine is so hopefully this fixes it

Psalmbird 4 years ago

Oh my goodness I could just kiss you!!! I've had cold air coming from my registers all day. I saw my light indicator, Googled what to do and your fabulous page with pictures and perfect instructions have saved me $100's!!! Thank you, God bless you!

Allen 4 years ago

Thank you so much was exactly the problem

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Armchair Builder 4 years ago from Hawaii

Great hub...even if you don't want to do the work, it's still good to know enough not to be taken for a ride by the repair guy!

Kelly Frame 3 years ago

Saved my life with that picture! Yay, had an outage and furnace is now working! Thank you so much!

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Pcunix 3 years ago from SE MA Author

A picture is worth 1195 words (in this particular case)

lsburk 3 years ago

Same thing after power outage and no repair man wanted to come out in the storm to fix it . Found the switch under a layer of dust and tada! Now I need to shop vac the dust out. Thanks

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Pcunix 3 years ago from SE MA Author

Yes, power fluctuations can confuse these things.

Lori 3 years ago

Thank you so much for this! Hubby was out hunting and I awoke to a freezing cold house. First, flipped the breaker off and on~just as you did and heard it cycle and thought~that was easy, only to have it go off again!! I ripped the thing apart and saw the green flashes, however, had no idea what to do about it. A google search brought me to you and my heat is now working. Hoping it is not indicative of a bigger problem, however, we had a power outage yesterday for two hours and I am wondering if that caused the problem. Also, had the lights flashing low battery on my control panel. I replaced those as well. I am keeping my fingers crossed~and leaving all the panels and screws off for hubby to deal with when he returns from the woods. Thanks so much, and the pictures were a fabulous help!!!!

Dave 3 years ago

These furnace companies rip you off to push a little switch I'm on a very fixed income I live off of Social Security disability I can't afford these outrageous repairs Thank you very much I hope this fixes my problem!!!!

Susan R. 3 years ago

Thank you! Came home and my furnace wasn't working. My husband was at work and my dad and I couldn't figure out what was wrong. Sure it was an ignitor problem. Husband said, "look for the green light!" So I googled and came up with your post. Thank you!! Baby and I are now warm and toasty.

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Pcunix 3 years ago from SE MA Author

I am really surprised at how many people have been helped by this. Such a simple thing, once you know the trick.

Ours has never acted up again, but if you DID have this recur, you should definitely get apro in to look at it.

Windchyme 10 months ago

OMG....THANK YOU!!!!!! I am on disability and while my parents help as they can...they are retired, not a lot of money flying around for calling out a tech guy especially when it is right after Thanksgiving. My furnace did JUST what yours found the little reset switch you showed (and I wouldn't have even seen it if not for you)..pressed it...turned the furnace off and on.....started up like a charm...working perfectly now. WHEWWWW!! You saved our bacon....THANK YOU!!!!!!!

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Pcunix 10 months ago from SE MA Author

I'm happy for you, Windchyme!

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    Tony Lawrence (Pcunix)759 Followers
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    I was born in 1948 and spent most of my career as a self employed computer trouble shooter for Unix systems.

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