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Pancho529 11-07-2011 12:11 PM

Furnace Troubles
I'm having issues with my furnace. It's an American Standard model number TUE080A936K2 and serial number P022W942G. It gave me some trouble (would not ignite) about a year ago. I disconnected and reconnected the wires/plugs and it worked fine. I'm having the same trouble again and tried the same solution. It worked, but only temporarily. I noticed the igniter would come on and light the burners, but then the burners would go out after a minute or two. There is a code showing (4 flashes) indicating open high limit device and the igniter does not come on now. It will come on if I turn off the furnace and disconnect/reconnect the plugs, but it gives me the same issue. I replaced both high limit switches on my furnace and I'm still having troubles. I don't think you can really install the fuses incorrectly... Does anyone have any advice?


Doc Holliday 11-07-2011 01:18 PM

sensor or ignitor. you can clean both with a scouring pad or emory cloth.

btw, does the wiring diagram only show two limits?

Pancho529 11-07-2011 03:17 PM

I believe there's only two limits. Is there any way to determine whether its the sensor or the ignitor? I don't believe there's a bad connection so I don't think cleaning them will do much.

BTW - I revised my initial post to be a little bit more thorough.

Pancho529 11-07-2011 03:22 PM


Originally Posted by Doc Holliday (Post 765846)
sensor or ignitor. you can clean both with a scouring pad or emory cloth.

btw, does the wiring diagram only show two limits?

Why would the burners go out after being on for a minute or two?

Quick update, I turn off the thermostat and the furnace itself. I disconnect and reconnect the wires and try to get the ingiter to light, but it will not. The diagnostic light is still flashing four times indicating open high limit device. I did replace two fuses or high limit switches and it has not resolved so I put the old ones back on.

beenthere 11-07-2011 04:12 PM

As long as the high limit is open, it won't light. need to fix the high limit problem first, then replace the high limit, then you'll have heat again.

Pancho529 11-07-2011 04:14 PM


Originally Posted by beenthere (Post 765981)
As long as the high limit is open, it won't light. need to fix the high limit problem first, then replace the high limit, then you'll have heat again.

What would that require?

beenthere 11-07-2011 04:18 PM

look to make sure no registers are closed or blocked off, and that the air filter and the A/C coil are clean.

Pancho529 11-07-2011 04:26 PM

Those are all open and everything's pretty clean.

beenthere 11-07-2011 04:35 PM

High limits go bad from cycling. Your duct work could be too small. or your blower motor could be bad or running slow.

Pancho529 11-07-2011 04:40 PM

This furnace is 11 years old. Therefore, I don't think the ductwork is too small. The motors are running fine, that's not an issue either.

beenthere 11-07-2011 04:41 PM

Could be the secondary heat exchanger is dirty. Something caused the high limit to open. Its a safety, not an operating control.

Doc Holliday 11-07-2011 04:42 PM

Most furnaces have a minimum of four limit swiches, older ones three, some apparently only two, why I asked if the DIAGRAM showed only two.

If FLAME SENSOR does not SENSE a FLAME then it shuts the furnace down. Why feed gas to a furnace if there's no flame to burn it? Make sense? That's why SENSORS sometimes need cleaning. It's only a small metal rod after all. Put fire to it hundreds of times and eventually it needs cleaning.

All furnace flame sensors read electrical current through the flame to ground, tested in micro amps. I'd also make sure you have excellent ground wiring and connections to and in the furnace, all the way back to the electrical panel.

Anyways, beenthere has this one and I have a serious headache, seriously, so I'm going back to sleep.

Thanks Bt.

beenthere 11-07-2011 04:51 PM

Seldom more then 2 limits for the heat exchanger.

Some have over heat limits on the blowers.

Doc Holliday 11-07-2011 05:04 PM

I went on a Lennox furnace service call, I think I posted a thread about it concerning the nitride ignitor and it being good, and the Lennox had four limits, that I could visibely see. Two Rollouts (one on top and one on bottom) and two limits, one on blower and one in exchanger. I didn't even look at the wiring diagram to see if there were any more.

On that old Carrier (like the old Consolidateds) that we discussed the control board on (that as well not being the problem in the end but rather no hot straight to line 1), over 20 years old, there were 3; one in the burner compartment that I literally had to pull the burners out to get to (and it tripped after new board and complete re-wire, I went back the next day and that's when I found that limit. customer called and said heat wasn't working), one in the heat exchanger and one down in front just before supply plenum, kind of where you'd normally see a fan limit.

That sound about right?

beenthere 11-07-2011 05:16 PM

Roll outs are not limits, they are roll out switches. Although they are temp sensitive, they are not heat exchanger temp limits. Some roll outs are in the limit circuit though.

I talk about them being different only because they are for/indicate a different problem. Which the Op could have.

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