microwave won't heat

My 18 month old whirlpool over the range microwave stopped heating a few days ago. Initially, it worked intermittently for a couple of days. Then, not at all. Everything else is fine and it sounds like it’s working properly, but doesn’t heat. Is there anything we can check without getting into the expense of repairmen, which are quite expensive where we live? It’s out of warranty period. Thanks for any help you might provide.