When I fire up my furnce after the burners come on the blower comes on and runs for about 3 minutes and then shuts off,then the diagnostic light signals that I have an open limit switch or aux limit switch is open can someone tell me what the problem is and how to fix it

Check to make sure your air filters are clean and all registers and return airs are open. If that is OK then you might have a out of calibration limit control.