Microwave Not Working – Fuse is blown

A blown fuse is one of the most common problems a microwave can have, but that’s just the indication that something has gone wrong. A blown fuse really means one of your electrical components has broken or failed, and the next step is to find out which piece that is. Here’s how:

What do you do know that you know the inline fuse is blown?

If the fuse is blown, that’s usually caused by another malfunction.

WARNING:  Do not attempt unless you have training in appliance repair.  Microwaves can have 2000 volts of electricity in the high voltage circuit and can easily lead to injury or death.  Be sure to discharge the capacitor before messing with anything inside the microwave.  If you do not how to discharge a capacitor safely, STOP RIGHT NOW.

Use a resettable fuse before testing so you don’t run through new fuses while testing primary malfunctions, but you can test with new fuses if that is more convenient. Then, you need to repower it: this means putting your microwave back together again so you can plug it in safely. Even though a microwave can run without the cover, you want to fully reattach it for your safety.

Isolate the high voltage circuit by removing one lead off the power supply leading to the transformer.  Once the power is back on, test it by microwaving something like a small bowl of water or anything that lets you easily test the heating power. In the most common scenario, your microwave will start up and run with no issue.  This verifies that nothing that takes just 120 volts like your door switches and electronic control is causing the issue.

WARNING:  Do not attempt unless you have training in appliance repair.  Microwaves can have 2000 volts of electricity in the high voltage circuit and can easily lead to injury or death. Be sure to discharge the capacitor before messing with anything inside the microwave.  If you do not how to discharge a capacitor safely, STOP RIGHT NOW.

Microwave Capacitor: Test the capacitor by first discharging it.  Isolate the capacitor and check for resistance on the most sensitive scale.  Your meter needs to be using a 9-volt battery.  Place your meter leads on the ends of the cap and you should see a quick rise and fall on your meter.  If the resistance stays high, the capacitor is stuck open and needs to be replaced.  If the resistance never changes, the capacitor is shorted and needs replaced.

Microwave Diode:  Isolate the capacitor and check for resistance on the most sensitive scale.  Your meter needs to be using a 9-volt battery.  Place your meter leads on the ends of the diode and you should see a quick rise and fall on your meter.  If the resistance stays high, the diode is stuck open and needs to be replaced.  If the resistance never changes, the diode is shorted and needs replaced.  This needs to be checked on both ends of the diode, and it should only rise from one end since diodes only allow electricity from one direction.

Microwave High Voltage transformer:  Transformers are very similar but we recommend you pull the wiring schematic to locate what wire does what.  From there, you will measure the proper resistance for each line when the microwave has no power.  If the resistance deviates strongly from these amounts, then the transformer is broken and must be replaced.

Microwave Magnetron:  There is no way to accurately bench check a magnetron.  Inspect the component for physical damage and rule out the other three components to determine if the magnetron is at fault.

If you want to learn more about diagnosing common electrical problems in your appliances or you’re looking for specific parts, check out Appliance Express for more information.