The fan on my convection oven keeps running after the oven is off. Does anyone have this problem or a solution?

Sounds like a stuck relay. If this is a combination microwave, hostile territory for the DIY. Once apart, the fan should have one wire connected to the main bundle of white wires, and another going to a little box with 3 more wires. Likely the thing is bad and not much of a job to replace once you identify it. Identifying it and replacing it will be more of a challenge if it is on a circuit board. Could push it into being more bother than worth. I am sure the landfills are full of stuff only needing simple repairs.

You should post this on the appliances page - I can answer you much faster. What kind of product do you have? Brand, model, wall oven, range, etc?