Hi, First of all, operating a microwave without an exhaust fan is a fire and possibly an explosion hazard. There are very high heats generated. The fan is operated by a control board relay. It can't be repaired. Failing control board needs to be traced, tested, and replaced. and it may not be the only problem.A schematic would be necessary but are not published due to danger of servicing high voltage circuits.
Unfortunately, it's not really a DIY project unless you're an electronics technician. If you are, the parts alone are in the $130 range. Here's the parts site.
Here's the situation. Much like televisions, microwaves are full of very high voltage (4000v-5000v) even when unplugged, and require care , schematic diagrams(unavailable to DIY servicers), and electronics experience to service. They are increasingly controlled by miniature pc boards and chips, impossible to repair, difficult to test, and expensive to replace. Also like televisions, once they're out of warrantee, parts and labor (if you can find them) can often approach or exceed the cost of a new machine. Therefore, like TVs, Microwaves have become cheaper to replace than to service, and service has become increasingly hard to find. You can check repair costs by clicking on SearsRepair for their plan. Cost could be weighed against cost of new unit with a new warrantee. It's up to you. I hope this answer is helpful if not welcome. Either way, let me know what you think. ed