Microwave Oven Repair 101

A white microwave, ready for use.
  • 1 hours
  • Intermediate
  • 50
What You'll Need
Volt-ohm meter
Replacement parts (as needed)

Microwave ovens are a testament to man's ingenuity and creativity. For someone to have the foresight to imagine that food can be cooked without ever being placed in an oven or near a fire is astonishing. The microwave oven is a marvel of science, available right in our kitchens.

So, you're here because your microwave oven isn't working as well as it used to or it isn't working at all. Many times, you can quickly and inexpensively fix the problem by yourself. You will need to have a volt-ohm meter, which you can pick up at most hardware centers. A notebook to record your process is a valuable assistant when it comes to trying to remember which screw goes where. As always, unplug the appliance before troubleshooting unless otherwise noted. Now, let's see if we can fix that problem.

Note: Before touching any internal parts, be sure to discharge the capacitor. The capacitor stores additional voltage and can hurt you even if the unit is unplugged. To discharge a capacitor safely, you will need the following: a screwdriver, a wire-wound resistor with a 2 watt-20,000 ohm rating, and a pair of jumper wires with alligator clips on the ends. Clip a wire to each end of the resistor. Clip one wire to the metal shaft of the screwdriver. Clip the other wire to one of the capacitor's terminals. Now, touch the other terminal with the tip of the screwdriver. There may be a small spark. If the capacitor has three terminals, do the same process with the middle terminal and each outside terminal.

Your Microwave Oven Won't Run at All

Unplug the power cord and check for voltage at the outlet. First, inspect the cord for any damage or burn marks. Because of all the safety devices in a microwave oven, any one of them could be the cause, but before you can look more closely, you will need to remove the outside shell of the microwave. Unscrew the screws underneath and on the back that hold the shell in place and slide it off.

Check whether or not the fuse is blown by removing it with a set of fuse pullers. Place it on a paper towel so it doesn't roll away, and then with your VOM set to RX1, place a probe on each end of the fuse. The reading should be zero. If not, replace the fuse with an identical one.

Your door switch could be the problem. Locate the door switches and remove the leads. With the VOM on RX1, probe the terminals. The reading should be infinity with the door open and zero with it closed. If not, replace it. Make sure to check both door switches.

Also, it could be a bad fan motor. Locate the fan and remove the leads. Once again, with the VOM set to RX1, probe the terminals. If the reading is infinity, then it is bad and needs replacing.

Your Microwave Keeps Blowing Fuses

Check the door switch as described above. The capacitor or diode may be bad. Discharge the capacitor as described earlier in the article, and then test it by removing the leads and setting the VOM to RX100. Probe the terminals. The reading should start in the low ohms and increase toward infinity. Reverse the probes and re-test. The reading should do the same thing, otherwise, you've found the problem.

To test the diode, disconnect the diode from both the appliance and the capacitor. With the VOM set to RX100 as before, probe the wires. Then reverse the probes and read again. You should get infinity for one reading and low ohms for the other reading. Another cause could be a faulty magnetron, but due to the sensitivity of that piece, it's best left to a professional.

Your Microwave Oven Cooks Slowly or Unevenly

Check the voltage at the outlet supplying power. If it is lower than 115 volts, there is a problem with your electrical service or breaker. A bad turntable motor may also be the cause. To check it, turn the microwave over onto its top and remove the bottom grill. Set the VOM to RX1, remove one lead from the motor terminals, and probe the terminals. If the reading is infinity, then replace the motor. The magnetron and the waveguide may also be the culprits here, but they need to be serviced by a professional.

Your Microwave Runs but Won't Cook Anything

For this problem, first, check the thermal cutoffs for both the oven and the magnetron. The thermal cutoffs are little disc-shaped devices with a wire connecting the two of them. Remove a lead and set the VOM to RX1 again before probing the terminals to look for a reading of zero. If the reading isn't right, then it will need replacement. You'll need to check both thermal cutoffs.

If these are okay, check the capacitor and diode as described above. The magnetron or transformer could also be bad, but they need to be serviced by a professional.

These are the easiest and least expensive repair situations for problems with a microwave oven. Any issues not covered here will require a professional in most cases. As always, have the make and model numbers handy when heading to the parts shop for replacements. If your microwave isn't the only appliance giving you headaches, this website has repair and information guides for many of them. Pick your next project, heat up a cup of coffee in your now-working microwave, and read on.

Looking to purchase a new microwave? Check out our Microwaves Buyer's Guide.

Dave Donovan is a freelance copywriter living in Atco, N.J. An electrician for 15 years, an injury forced him to pursue his true passion: writing.