The convection fan in my oven no longer shuts off when I open the door. Can I fix this myself (I am handy), or should I call a service person?

There will be a door switch somewhere around the door that should shut the fan off when the door is opened. It may have just fallen out of adjustment. Or if the actuator on the switch is bent, it would be best to replace it.

I would bet you money that its one of those little microwave style door switches, these are very common and only cost a couple of dollars. You find them in almost everything.