Top Reasons Why Microwave Is Not Heating

Microwave ovens have revolutionized the way many people around the world prepare and eat their food.

If your microwave is no longer heating properly, it’s important to stop using it until you have targeted the source of the trouble.

From defective door switches to a faulty magnetron, a lot can go wrong with a microwave. Always start with the most obvious and work your way up to find the root cause of your microwave not heating.

Reasons Why Your Microwave May Not Be Heating

Failed Magnetron

What is it: The magnetron is perhaps the most important part of a microwave and is responsible for the heat generated within the unit.

Magnetrons are oscillators that emit electrons from a hot cathode.

These electrons are emitted past resonant anode cavities, which ultimately generate the microwave energy responsible for heating your food.

Why it fails: A defective magnetron will result in your microwave not heating. There are a number of factors associated with the magnetron that could affect its performance. It’s best to start with lesser parts before replacing the magnetron outright.

In the following examples, you will see parts that can indirectly affect the performance of the magnetron. It’s always better to test them first and work your way in.

These parts are going to cost far less to replace compared to replacing a magnetron, so start small and test each one to ensure that they are functioning properly.

How to fix: You will need to check your magnetron with your multimeter to help you determine if the magnetron is working correctly.

You will first need to remove the housing around your microwave to access the magnetron.

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This is a large, metal, square-shaped unit that is usually visible immediately after opening the housing.

You should see a wire connection plugged into the magnetron. After unplugging the magnetron, connect the multimeter probes to the exposed terminals.

First, make sure that your multimeter is set to its ohm setting. After connecting the probes to the terminals, your multimeter should read less than 1 ohm.

Anything greater will let you know that the magnetron is defective and needs to be replaced.

Magnetrons are not able to be fixed. Anytime that you have a defective unit, replacement is your only option unless you decide to buy a new microwave altogether.

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Failed Thermal Fuse/Cavity Fuse/Thermoprotector

What is it: These serve to cut the power from the microwave in the event that it overheats.

This safeguard can prevent fire and other damages from occurring, so it is crucial that you make sure yours are all functioning properly.

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Why it fails: If any one of these units becomes blown, it can result in your microwave not heating. It’s usually easy to determine if this has happened.

The fuses will appear scorched and darkened, or the filament will be melted.

How to fix: Check the terminals of each using your multimeter. Your multimeter will need to be switched to its ohm setting before testing.

If the reading is close to zero, the fuse is good. If, however, it reads infinite, you’ll know that the fuse is blown and needs to be replaced.

Failed Door Switch

What is it: Most microwaves are equipped with 3 to 4 door switches. These simply let the microwave know that it is safe to start heating once the door is completely closed. 

Why it fails: A defective door switch can prevent the microwave from heating, even if the door is closed. As such, it’s important to remedy the failure so that your microwave will start heating properly again.

How to fix: Using your multimeter, check each switch to ensure that it has continuity. If one or more of the door switches has failed, you will need to replace the defective ones to restore your microwave’s proper function.

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Failed Diode

What is it: Your microwave’s diode is responsible for converting the alternating current power output of the transformer to direct current.

This doubles the voltage to nearly 5,000 volts. With sufficient voltage, the magnetron is able to be powered to where it can adequately heat your food.

Why it fails: The diode can become burned out, resulting in the magnetron not getting the necessary voltage it needs to power up and heat your food.

Fortunately, it’s usually easy to tell if a diode is burned out, as it will appear physically damaged.

How to fix: The microwave’s diode is located next to the magnetron and high-voltage capacitor. As such, you will need to remove the unit’s housing to gain access to test the diode.

Before testing the diode, it is important to first release any stored electrical charge within the high-voltage capacitor. Failure to do so can result in injury.

Using insulated pliers, touch each head to the capacitor’s terminals. Always be extra cautious that you don’t accidentally touch any part of the metal pliers when releasing the stored electrical charge.

You are now ready to test the diode with a multimeter. You will also need a 9-volt battery.

Simply hold the diode to one of the probes while touching one of the battery’s terminals. The other probe should be touching the other battery terminal.

Continuity should be visible in only one direction. If it shows continuity in the other direction, you know the diode has failed and needs to be replaced.

You can also reverse the leads to test continuity in both directions.

Failed High-Voltage Capacitor

What is it: This device works in tandem with the diode by converting the alternating current to direct current (AC/DC). Additionally, it assists in doubling the voltage of the magnetron so that it can be powered up to heat your food.

Why it fails: A defective high-voltage capacitor will cause the entire high-voltage circuit to stop working, resulting in the microwave not heating.

How to fix: The high-voltage capacitor is located near the magnetron, so you will need to unplug the microwave and remove the housing to be able to access the capacitor.

You will need a specific VOM meter that is equipped with the ability to test capacitance. Before testing, however, it is important to first release any stored electrical charge. Failure to do so can result in injury.

Using insulated pliers, touch each head to the capacitor’s terminals. Always be extra cautious that you don’t accidentally touch any part of the metal pliers when releasing the stored electrical charge.

If the capacitor has failed, you will need to get a replacement for your microwave, as this part cannot be fixed.

Failed High-Voltage Transformer

What is it: The high-voltage transformer is responsible for powering up the magnetron.

Why it fails: It’s pretty easy to target this issue, as you are likely to notice an electrical arc accompanied by a burning smell.

How to fix: Checking the high-voltage transformer is easy, as you simply need to examine it to determine if it is working properly. If you see that your high-voltage transformer is producing sparks of electricity, cease use of the microwave until you can order a replacement transformer. Continuing use could result in even more damage to the microwave or lead to a fire.

If you don’t notice any visible signs that it isn’t working correctly, you can test the transformer with a voltmeter if you believe it is to blame for your heating issue.

With the microwave unplugged, disconnect the high-voltage transformer from the power source. Since you are also dealing with the high-voltage capacitor, you will need to release any stored electricity.

Using insulated pliers, touch each head to the capacitor’s terminals. Make doubly sure that you don’t accidentally touch any part of the metal pliers when releasing the stored electrical charge.

Using the ohm setting, use your voltmeter to test the terminals for resistance. It should read somewhere in the range of 50 to 70 Ω. If you notice that there is a significant difference in this range reading on your meter, you’ll know that the transformer is defective and needs to be replaced.

You can also disconnect the input terminal’s leads and test them using your voltmeter as outlined above. However, this time, your meter should read zero ohms (or close to it). Any variance signifies a defective transformer.

Failed Main Control Board

What is it: The main control board is what allows you to set the cooking time, adjust heat levels, and change other settings, but moreover it controls everything in the microwave.

Why it fails: Although uncommon, a defective main control board can result in the microwave not functioning properly, including irregular heating performance.

How to fix: The entire unit will likely need to be replaced if it is defective. However, it’s important to note that you should save replacing the control board for last. You want to make sure that you exhaust all other possibilities before investing in a new control board.


Repairing a microwave oven poses serious risk or injury if you don’t take your time and follow proper safety protocols. Always make sure that your microwave is unplugged before attempting any troubleshooting or when replacing parts. Protection Status