Model #UMV1152CAB, the complaint on this microwave was it would work for short periods of time, but if you tried to use it any more than about 3 minutes, it would stop working and then would only work again after about 20 minutes. Not a bad problem if you only heated your leftovers, but it could become annoying when you really needed it. So lets follow along and see why this microwave seems to go on strike.
It Really doesn't Heat
Just to make sure we are all on the same page, microwave ovens don't actually use heat to cook, but instead use radio waves to stimulate the atoms within the food causing friction because the atoms don't want to move. Heat is then generated from this friction resulting in our hot pocket being hot after we microwave it. Now the heart of the microwave is the magnetron (radio transmitter) which produces the microwave energy that is broadcast into the oven cavity. While the cooking process is happening, very high voltage is being applied to the magnetron and aside from producing radio waves for cooking, the magnetron itself will give off heat (not for cooking) that must be removed in order to keep the electrical components operating properly. For that reason, microwave ovens will have a cooling fan that turns on when the start button is pressed.
Magnetron's don't like Heat
Because this heat can become excessive and result in permanent damage to the microwave, there is usually one or more TOD's (Thermal Overload Device) incorporated into the microwave wiring that will open the heating circuit in the even the magnetron or the oven cavity itself gets to hot. Since most TOD's used in microwaves are a one-shot device meaning once opened, they will not reset under normal conditions, the electrical components are very dependant on the cooling fan(s) within the microwave to keep the heat to a minimum.
A Model Specific Issue
This particular microwave being an over the range model which incorporates ventilation with cooking has a blower which is used as an exhaust fan, and at the same time, a cooling fan for the magnetron. The problem being is when the customer (installer) makes a change to the blower assembly to another exhaust position (and they don't read the instructions) the part of the blower that keeps the mag cool, isn't pointing at the mag anymore. So now without adequate cooling, the magnetron gets hot and the TOD mounted to it will open up the heat circuit (it's rated to open at 150 degrees F) thus making the unit stop working. Now the fortunate thing for the customer is in this particular model, the magnetron TOD WILL reset itself once it cools to 60 degrees F and the unit will start working again.
Fixing the Problem
The solution to this overheating problem is to adjust the right side of the blower assembly so it is pointing into the air duct that cools the magnetron. Once the cover is removed, it is quite noticeable which way it should face (and the diagram on the blower helps too) and as soonas it is turned to the proper position, the microwave will start to function properly again.