4 flashes on troubleshooting display says open high limit device, where is this device located on furnace?

Had a Trane XL90 two-stage high efficiency gas furnace installed in 02'. Open limit switch diagnostic L.E.D. came on last night and the electrical schematic is completely corroded need help finding the switches. Thermostat said 43 this morning.

I have a Trane Gas Furnace, model # TUE120A96OK3, that is 6 years old. I am having an issue with it where the burners will not stay lit. I check the red LED light diagnosis, and it says that it is having an "open high limit device" error. I thought that I could possibly reset or replace this to fix...

What are the proper settings for this particular model. The fan stays on way to long after the heat/fire shut down, which then causes the house to cool down quickly.

Furnace is about 13 years old... we just replaced the ignighter in it last night because it was running through all the steps to run except ignighting. It fired right up and ran for about 7-8 hours... now it won't ignite at all (it is running through all the steps it needs to except for...