Have a Trane xv80. Recently we have issue in furnace heater. It stops after a few minutes of heat call and fails due to highr limit device failure. I replaced the high limit switch recently. It is closed before heat call and opens as expected in case of overheat. The real problem is blower fan does not start along with heat call and it makes the heat exchanger overheated. The fan is supposed to run 5min hourly. It runs well only when the system is cooled down. Any clue or idea is welcome. Thank you.