We have a Trane furnace #TDE080A945K2 that hasn't been heating. It is just blowing cold air. We reset the furnace as instructed in the manual and after turning it back on it lights but then shuts off again after a minute or two and won't light again. We have cleaned the filter and the thermocuplar is working because it gets red. Any ideas on what it could be? We have called a pro to come look at it tomorrow but wanted to have a heads up on ideas?

Problem could be a dirty or bad flame sensor. It will look like a small metal rod right in the burner area, if it does not sense a flame. It shuts gas valve down until reset or cleaned. Usually will shut down once the hot surface ignitor stops glowing. Hope this helps.
Brian

Read through the sequence of operations of a gas furnace in the sticky at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html

Usually if the furnace doesn't ignite, the blower won't come on. Could be a bad temperater sensor or on an older furnace, a limit switch. Could be a bad control board, $$$$$$$$$$