My Trane XR80 furnace is showing a flashing code of 2. The manual indicates it as external lockout. Any ideas as to what this means and how to repair it?

I have a Trane XR80 furnace. I have noticed that it is now running constantly (with heat for a while and then no heat). I checked the diagnostics code and it is now flashing 2 times. The code indicates it as an 'External Lockout'. Anyone know what that means and how it can be repaired?