I have tried all of the instructions on the front of the furnace about turning the thermostat down, turning off the furnace, waiting 5 minutes and then restarting. The furnace still only blows cold air. Help. :(

Hi I have a Trane XE80 that I'm trying to fix. LED on the control board flashes 4 times indicating a Hi Limit switch open. Where is the hi limit switch? I've metered all of the sensors that I could find and nothing shows open. Does anyone have a part number for the switch? Blower comes on...