Model AUD060C936K1
I'm here in Colorado and just moved into my home that has an American Standard Freedom 80 single stage furnace about 13 years old according to the paperwork the last owner left behind. This cold front has moved in and I'm without heat so thanks in advance for all your help.
I randomly am getting 4 flashes indicating the high limit switch. There is no pattern to when it might start flashing and begin blowing cold air. I've replaced the high limit switch which opens at 220 and closes at 190 with no help. If I turn off power to the furnace momentarily it might work continuously for an hour it might go into fault right away. The one 16x20x1 filter has been replaced and I've cleaned everything I am able to see including vacuuming all the dust off the blower fins. It worked fine through the night last night and was off until I came home today when I turned it on and it ran for almost 90 minutes with no issues. About an hour later I turned it on and less than 10 minutes later I'm feeling the cold air and sure enough 4 flashes. One other scenario was on Saturday morning when it immediately when into fault after initial warm up when the blower started, I turned off the furnace and put in the old high limit switch and it worked through the morning, sat idle though the day, and worked all night. Sunday morning after a couple cycles of heat it went into fault again and I put the new switch back again. So I'm also not sure about the switch itself. All registers are open and all my ducts have been cleaned. Highs in the teens and twenties and lows in the single digits here for the rest of the week so again I appreciate any suggestions and help.

 

Welcome to the forums!! The link you provided shows as invalid. As you are probably aware, the high limit switch opens because of an over temperature condition. One reason could be the blower is not moving enough air. Have you checked your vent pipe to make sure there is nothing inside like a bird's next, dead animal or other object. Also, if you have an a/c evaporator coil on your furnace an often overlooked thing is a clogged/dirty coil that restricts airflow and causes limit switch to open. Also, make sure none of the return are blocked off by furniture or other object. I have been on calls for the limit switch tripping because the homeowner moved in and was unaware there were air filters behind the return air grates and never changed them. Once they got dirty enough the furnace overheated and tripped the limit switch.

 

Thanks..... Well I took it apart so I could access the coil and they looked cleaned on both sides. I went to Home Depot for some foil tape and got some foaming coil cleaner and cleaned them for good measure anyway. The vent gets nice and hot but is clear to the roof. So after I put everything back together I started it up then got about 2 minutes of heat then burners off and 4 flashes. I put the old high limit switch in to see what would happen and I got heat for the full cycle up to my requested temp which took about 30 minutes. After about 10 minutes of sitting idle the furnace started its warm up then went into fault 4 flash and blowing cold. Seems to me like the swapping of the high limit switch tricks it for a cycle, I'm guessing but I really don't know if there is anything else I can do. Any other thoughts??

 

Here are a couple of documents that may provide some useful information:

http://www.trane.com/commercial/uplo...1112/tud_c.pdf
http://c.searspartsdirect.com/mmh/li...L/L0903222.pdf

One of the documents is for a Trane but it is the same furnace and data as yours. It shows the blower speed taps and airflow charts.

Does the blower seem to be putting out a strong air flow through the supply vents into the rooms?
I have seen a couple of cases where the insulation inside the furnace came loose and was sucked against the sides of the blower restricting the air flow. Also, a weak run capacitor on the blower may prevent the blower from coming up to speed.

It would be a good idea to check the temperature rise to see if it is within limits. Maybe the blower speed is not set properly and is causing the furnace to overheat. The temperature rise can be checked with a probe type thermometer like restaurants use to test the temperature of the food. The data plate usually lists the the temperature rise.

 

Thanks again.....good air is coming through all vents and the temp rise is about 50 degrees with a limit of 30-60. Another oddity I found is that I can run the furnace multiple cycles with the top panel removed with no problems. I just wanted to get a better look and with the panel off it ran for over 2 hours. I put the panel on and it went into fault immediately one time and after one minute another time. I don't know if this is telling me something about the pressure or what but it is always 4 flashes. I'm puzzled and not sure what to do next.

 

That is some odd behavior. Have you tested voltage at the limit switch to see if it is actually opening when it throws the 4 flash code?

 

Well it's starting to get cold here in Colorado so I figured it was a good time to troubleshoot this some more and put it to bed. Even though everything tested fine previously I never didn't think one of the parts could still be bad. The situation got worse where it would immediately go into 4 flash fault so I was determined to fix this asap. I started by bypassing/jumpering each of the 4 temperature limit sensors to see if there was any change. When I got to the second flame rollout switch it started working. Now I'm looking for one of those. I attached a picture of when I finally could breathe easy, that omg moment, finally. Thank you again firedawgsatx!!! !!
Wolf

 

I'm sure firedawg would appreciate the thanks. I haven't seen him around since July.

You tell by the page views of this thread that it's a common problem and others are looking for a solution.

Just as a further note.... usually the air flow thru the furnace doesn't affect the flame roll out switches. In looking at one of your pictures.... the flame looks like it might be overfiring. It wouldn't hurt to get a gas tech in to check the heat exchanger and the gas pressure to the burner. If the exchanger is always running hot it can shorten its life.

The flame roll out switch should be Trane part # SWT02173 which is a replacement for the old part numbers SWT01978, SWT02176.

 

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The flame rollout switch is pretty much NEVER the problem.


What you probably have is a flame rollout switch that is being overheated and is opening for a reason.

That could be burners that are overfired, an inducer motor that isn't coming up to proper speed or some other defect.

 

Dangit, I appreciate the response and you are right. I'm troubleshooting this again. It worked good for a couple days but now it's going into 4 flash fault immediately.

 

Heh, heh! Furnaces can be cussed things, right? I encountered MANY of those during my years as a gas appliance repairman!

If my last post doesn't provide sufficient guidance, describe what you are doing and finding.

 

Since the thing was going into immediate 4 flash high temp fault without even firing I thought the next thing would be the IFC board. I replaced that yesterday and so far so good.