Here's a couple of ideas. The wires that connect to the limit switch can be faulty , a break or a bad connection in these wires at the connection to the board or the next device in line can make it look like a fault to the board when it really isn't. If you have a volt meter you can check this when the furnace goes into fault by disconnecting the wires on the limit switch and measuring the continuity across it. ya see the first thing you need to determine is if it is actually going out on fault or not , my hunch is that it is not.
Another scenario could be that it is going out on fault, if this is the case you probably have a problem with the heat exchanger and are not moving enough air between the cells where the limit device is located. This could be due to a plugged up cooling coil or some other obstruction like insulation falling into the furnace and blocking air flow through that exchanger.
In an extreme case your heat exchanger can have a hole in it, this however is not likely because you would experience other problems with it as well.
Those are the things to look for.
I am going to go home and check those things. I am at work right now.
Let me know
The insulation in the blower compartment is for noise reduction only. So no you don't have to replace those. Was that the problem ?
Does your meter show 0 for an open circuit or for continuity some are different. If you touch your leads together do they read 0 ? Probably reads 0 for no ohms no resistance which means the switch would have been closed and not actually tripped. Let me know,
When I touch the tips together I get a zero reading. It is a Craftsman brand if that helps.
OK, that means that the limit switch is not open when you are testing it. How much time do you think passes between the time you are measuring it and the time that it goes off on fault ?
This AM it went off on fault and I got out of bed and checked it immediately. I would say a few minutes passed, maybe 10. I left the power on for the unit and unplugged the wires from the limit switch and tested it. As I said, the reading was zero. Yesterday I removed those insulation panels that had come loose, so that wasn't the problem (or at least not yet). When I was working on it in the past couple of weeks, I replaced the Ultra Allergen filters I had in there with a cheaper , less restrictive, pleated filter to try to increase air flow. Also, I backed off the gas valve to make it leaner and reduced the temp of the air coming out of the vent by 20F. It sounds like the limit switch isn't even bad? What could cause it to go out like that? It sometimes will work for a few days without tripping. After our conversation yesterday and removing the panels and tightening the clips on the connections, it tripped this morning after it was running for a while to heat up the house. It seems to happen after the unit has run a long while to heat the house and then on the next cycle or so after that, it trips.
Let me know your thoughts.
I am heading to my house now and will be there in 30 minutes so I can try whatever you suggest. Thanks again for your help.
I think you have to look for a loose wire on the terminal board or on the limit switch circuit board. Other than that the board is just not reading its inputs properly.
When you read the ohms on the limit switch that is the switch its just the wires that go to the circuit board. The limit switch looked something like this ? but you would have only seen the front of it. Let me know.
Unfortunately it sounds like we are headed toward replacing the circuit board. If that switch is closed and the wires running from it to the control board are intact then its just the circuit board that is failing.
Being a Trane unit it will be hard to get. You may want to have a pro do it.