2002 Saab-- Start vehicle put in gear and whithin 10 seconds I feel the tranny come out of gear. Scanner shows 0 diadnostic code.

Stick or auto? If stick, probably synchronizers. If auto, could be a number of things.

Neutral Safety Switch may be worn and need replacing.

If it's a manual transmission... worn sycro hub / or engagement dogs on the gearset. Had that happen on several transmissions over my 1.5 million miles driven on cars and on motorcycles.