Troubleshooting says "restricted or plugged furnace condensation drain" - 1. Remove drain clamps 2. Flush or clean 3. Reinstall clamps.
I could really use a diagram. I'm just not seeing it.

I got a Trane XR80 installed in 2005. It started to have problems since January 2011. Sometimes I got heating while other times I got none. I searched the web and got information on how to read the error code. It is 4 flashes, i.e. the Limit Switch might report over heated somewhere. Then, I followed the instruction to change filter, clean up, etc. It worked for a while and stopped working this year.

Sometimes, I could still get heating for a while without any problem. So, I believe that it is not due to a real problem. It is rather a defective parts. The cheapest parts is the Limit Switch. So, I decided to replace it. I found on eBay the following parts:
Trane 3" Limit Switch L145-30F C340056P15 SWT01277 sold by plasticnow for $12.00

After installed this Limit Switch yesterday, the heater works excellently. Now I have heating whenever I need it.

The limit switch is a safety device. Why did it go bad if the furnace is running correctly the limit should never be used yours has been used so many times the limit failed. You need the look further into this to solve the real problem.