Microwave Not Heating? How to Fix It

If your microwave is not heating, it is most likely because of a faulty magnetron, high-voltage capacitor, or diode. These three components are responsible for heating and cooking the food. The microwave will likely still work if one of these components fails, but it will not be heating or heating at 100%.

Depending on the microwave, it is unlikely to be working at all if another component is responsible. However, it is possible that a blown thermal fuse or Thermoprotector, or a defective door switch or control board, could be causing the heating issue.

We will outline how the heating issue can be fixed; however, repairing a microwave is not as straightforward as repairing other household appliances, so make sure that you read the safety warning below first.

Safety warning

Due to the substantial safety risk, repairing and testing internal microwave components is best left to a trained microwave technician. The microwave’s high-voltage capacitor can contain a potentially fatal amount of electricity, which can remain in the component long after the microwave has been disconnected from the power.

Therefore, the high-voltage capacitor must be discharged before it is safe to touch or handle any of its surrounding components. The high-voltage capacitor can be discharged by touching a metal blade with an insulated handle to it. Electrical safety gloves should be worn, and extreme caution must be taken not to touch the metal while discharging the capacitor.

Defective magnetron

The microwave’s magnetron produces the microwaves that heat and cook the food. If the magnetron fails, the microwave will not heat, and it may also cause a fuse to blow, which can disable the microwave completely.

There are several reasons a magnetron may fail, most of them relating to the microwave being too old. Therefore, while you can replace the magnetron—it cannot be fixed—you may decide to buy a new microwave instead.

Defective high-voltage capacitor

The microwave’s high-voltage capacitor converts alternating current to direct current to create the high voltage that the magnetron needs to cook the food. If the high-voltage capacitor is defective, the magnetron will not get the voltage it needs, and so the microwave will not be heating. Make sure you have read the safety warning before attempting to test or replace the capacitor.

Defective diode

The microwave’s diode works in much the same way as the high-voltage capacitor. If the diode fails, the magnetron will not get enough voltage to heat and cook the food.

If the diode fails, it is usually because it has burned out. Therefore, it is often easy to tell that a diode needs to be replaced just by looking at it. If it appears to be burnt out, it will need to be replaced. If you cannot tell by looking at it, it can also be tested with a multimeter to determine if it has failed.

Blown thermal fuse or Thermoprotector

The microwave’s thermal fuse or Thermoprotector is a safety feature that shuts off the power to the microwave if it overheats. If the component has blown, it cannot be reset and will need to be replaced.

Replacing the thermal fuse or Thermoprotector is usually a relatively easy procedure, except for the high risk when accessing the microwave caused by the high-voltage capacitor. If you can find a diagram of your particular microwave model, you may be able to locate the component and replace it if it is not located near the high-voltage capacitor, but it is still a potentially fatal risk.

Defective door switch

Depending on the type of microwave, there may be one or a few door switches that will not allow the microwave to operate if the door is not closed. A door switch can wear out and need to be replaced, or it can come loose from its mounting and need to be adjusted. If you decide to access and test the door switches, you can check that they are secured correctly before removing them to perform the test.

As with the other microwave components, it may be possible to access and replace the door switch, but depending on where it is located in your particular microwave, the high-voltage capacitor could pose a serious safety risk.

Faulty control board

Although unlikely, a faulty microwave control board could be causing the microwave not to heat. Make sure you have checked the components above before considering the control board as the source of the heating issue. Replacing the control board can be expensive, so you do not want to misdiagnose the control board as the problem.

It is also difficult to test the control board, and it will most likely require the knowledge of a trained microwave control board technician. However, it can be inspected for signs of burning or shorted wires. If there is obvious damage to the control board, it should be replaced.

How to determine if a component has failed or a fuse has blown

Follow these steps to check and replace the above components:

1. Disconnect the power to the microwave.
2. Access the microwave by removing its casing. Depending on the microwave and the component, you may need to remove the control panel or both.
3. Discharge the high-voltage capacitor (strongly recommended).
4. Locate and check or remove the component.
5.  If applicable, test the component with a multimeter for continuity.
6. Replace a defective component.

When testing the high-voltage capacitor, it should be tested with a VOM meter that has capacitance testing capability.

When testing the diode, it needs to be tested with a multimeter that uses a 9-volt battery or by using a 9-volt battery with a multimeter. With one multimeter probe touching one of the battery’s terminals, the other probe touching a diode terminal, and the remaining diode terminal touching the remaining battery terminal, the diode should only show continuity in one direction. If there is no continuity in both directions, or there is also continuity when the probes are reversed, the diode has failed and will need to be replaced.