Ok guys, sorry if this is a repeat but I couldn't find an answer that I was satisfied with.

I have a Thermal Zone GU100M16B-S gas furnace. It was put in about 4 years ago and I have had problems from the start. The PC board has been replaced twice (Nordyne 624631-A) , along with the gas valve and the pressure valve.

This year I am getting a one flash code on the board which means either a limit switch is tripping or a load on W. I have had two "pros" out to see what they can do and they haven't helped but were glad to charge me. The thermostat has been replaced and the code is the same but the sequence of events is different.
It use to be that it would take several (8-10) cycles before the temp would come up to the set temp. Now it takes 3-4. With the old Tstat the inducer motor would stay running and the burner and blower would kick out. Then the burner would light and the blower would kick in. Now the blower motor stays on with the inducer and the burner kicks out.

From what I have been reading I think the culprit is the main limit switch since none of the manual resets are tripped. I tried removing the blower door so that plenty of air was being drawn through. Thus also eliminating the filter being the problem, it didn't help. So today I turned the power off and put a wire across the limit switch with the lugs still attached. I am now thinking that I should have removed the lugs from the switch and hooked the two lugs together. The burner still kicked out BUT there was no code flash on the board. Again this leads me to believe I should have removed the lugs from the limit switch.

So with that info where can I look or guide a repairman to look at?

You might call them and se if they can steer you to a service manual.

Thermal Zone - Home (http://www.thermalzone.com/index.html)

Well, after several people and some strange conversations it looks like I am out of luck for a service manual. I'm not sure but I think it might have something to do with not being an HVAC contractor.
I guess I will let the furnace break again and then just replace the entire thing. I am tired of dealing with it! With spending $400-500 a winter I could have bought a new one.

Actually I am not informed about that company since they are a very small producer of items that sell in this area. In fact I do not believe there furnaces are available in this area under that name. Sorry about no help from the manufacturer.

From what I understand Thermal Zone did indeed go bankrupt, I think the year after my furnace was installed. Nordyne picked up the line but didn't manufacturer under that name. I have seen a couple schematics that have a cross reference to Nordyne or other parts. For instance, the PC board is now a Nordyne. Neither Nordyne or the web page that you linked to have any Thermal Zone information... or that is what they say.

ThermalZone - TZ81UH Gas Furnace (http://www.thermalzone.com/products/gasfurnaces/TZ81UH.html)

What?? Here is a current furnace they sell. This is linked right off the link I posted above. They are in business I believe

Here is the thermal zone warranty that is posted.

http://www.thermalzone.com/warranty/ThermalZone_Warranty.pdf

You are correct they probably do not want to deal with home owners just like the rest of the manufactures. That is why they have a dealer base.

Sorry you are having a hard time of it but the next time try the info in the link below to prevent problems.

Yes, Thermal Zone is back up and running but they tell me that my furnace was built by Nordyne. Which I do know bought the old Thermal Zone.

Yep thermal zone is not building there own unit anymore just like many of the other brand names out there. Glad you got it going.

Here is a link (http://en.allexperts.com/q/Heating-Air-Conditioning-696/Thermal-Zone-AC.htm) for you.

Still having problems with the furnace but since I have heat I'm not going to mess with it. Like I said earlier, let it break and then I will replace it. I have plenty of heaters that I can use.