fan stays on for about 30 mintues and then displays an F3?

My kenmore Elite microwave model 721.89823500 fan stays on after door is open and closed. It will run for about 30 minutes and then turn off with an f3 on display. Also the convection oven part of it will not heat. Also some times it will display oven hot. bought it new from someone but it was never installed it was still in the box opened but not new. Is there an way to test to see what the problem is some kind of diagnostics that can be ran from the front panel or anything else....... thanks for anyhelp

dave
May 2007