Maytag microwave is not heating and the time runs back fast.

Hi, it sounds like the magnatron has failed. This is the component that produces the microwaves that heats the food.

The cost of the magnatron is about 95% of the cost of a microwave oven. Most people don't, for this reason, replace the magnatron, they replace the whole oven.

There is a possiblity that the circuit that provides power to the magnatron is faulty and you need to take it apart, find the proper electrical leads and test them with a volt meter. To do this you have to remove it from its mounting and take it appart. It is a fairly lengthy process however.............