whirlpool model #665.686 00891 just plain quit working.

Hi @stevenrobi59391

Here's a link to a parts supplier that is useful.

Go to the Interior and Ventilation parts diagram Item # 28 to view the location of the fuse. It is a 250V 20A ceramic cartridge type fuse (part #W10138793). Search online for the part number if you wish to get the OEM fuse in lieu of a generic similarly rated replacement fuse

There's always a reason why a fuse fails and the reason for it will have to be found as usually they don't fail of their own accord. So if it has blown maybe order two or three just in case you just replace it first without checking as the replacement may blow as well. - see the safety warning below.

One common reason for microwave fuses failing it due to the door interlock switch contacts arcing and then fusing together. The switch contact migration (scroll down to 1st image on page) is due to continual misuse by the user over a period of time. The misuse is that instead of stopping or pausing the microwave during the cooking process to check or stir etc by using the control panel buttons i.e. pause or stop, they simply open the door to stop it. This causes arcing to occur across the switch contacts that eventually results in the contacts fusing together. The fact that the oven stops working when the door is opened is a safety design feature that is there to prevent harmful microwave radiation escaping from the oven cavity when the door is opened with the oven turned on. When the door is closed, the oven acts like a Faraday cage to keep the radiation inside.

A way to know if the door interlocks are OK is to use an Ohmmeter to check that all the door interlock switches (there are 3 switches) contacts are operating and releasing OK "electrically". Do this with the power disconnected - see the safety warning below.

If a switch is faulty see items #10 & #13 Interior and Ventilation parts diagram to find the part number and then search online for the part number only to find suppliers that suit you best.

If the switches are OK then it may be a faulty wiring, HV transformer, HV diode or worst case a faulty magnetron. Here's a link that may help with the testing of components in a microwave oven.

Be safety aware when working in microwave ovens. The HV capacitor can store >5000V DC for months, even if the power has been disconnected for this length of time. This amount of voltage can seriously injure you. The HV capacitor needs to be correctly discharged as soon as it is accessible after disconnecting the power to the oven, then removing the cover from the oven and before any further work is started inside. Microwave ovens can be dangerous to work on. If you don't know what you're doing, don't do it, it's not worth it!