How to Repair Microwave Oven Not Heating

Microwaves are a staple in most people’s kitchens. A fantastic invention that we tend to take for granted. If the microwave stops heating, it is easy to think you may need a new microwave. However, in most cases, it will be easier and cheaper to identify and replace the defective part causing the issue.

Typically, if the microwave is not heating, a door lock or blown fuse will be causing the issue. If the fault is more serious, the high-voltage capacitor, magnetron, or diode may need to be replaced. To determine the cause of the heating issue, these parts will need to be troubleshot, starting with the door switches.

Safety Warning

Repairing a microwave is more dangerous than repairing most other household appliances. The high-voltage capacitor inside the microwave can store a lethal amount of electricity, even after the microwave has been disconnected from the power for several months. Before repairing internal microwave oven components, the capacitor should be discharged.

The capacitor can be discharged by touching both the positive and negative terminals of the capacitor with a metal screwdriver blade. Precautions must be taken to insulate yourself against electrocution. A screwdriver with a rubber handle, or needle-nose pliers with rubber handles, can be used to discharge the capacitor. Due to the risk, repairing the microwave and discharging the capacitor should be left to a trained microwave repair professional.

1. Defective Door Switch

Most microwave ovens have 3-4 door switches that only allow the microwave to operate if the door is closed. This is a safety feature to stop harmful microwaves from escaping the microwave when it is in use. If a door switch is defective, it can prevent the microwave from heating, even if the door is closed. Door switches can become defective due to wear and tear, or they may fail to work correctly if they are not held in place securely.

Most door switches are activated by the prongs on the door, which should activate the door switches when the door is closed. If a door switch has moved or is not secure, the prongs on the door may be unaligned and failing to activate the door switch.

To access and check the door switches:

1. Disconnect the power to the microwave.
2. Remove the microwave cabinet.
3. Remove the control panel to access the door switches.
4. Check if a door switch is loose and not connecting with the door prongs.
5. Remove the door switches from the microwave.
6. Test the door switches for continuity (a continuous electrical path) with a multimeter.
7. Replace a defective door switch.

2. Blown Fuse

A microwave contains several fuses, such as a thermal fuse, a cavity fuse, and a thermoprotector. The fuses are all designed to keep you safe by cutting power to the microwave if it becomes too hot or there is a power surge. If a fuse is blown, it cannot be reset, so it will need to be replaced. A multimeter can be used to test the fuses for continuity to determine if they need to be replaced.

Microwave fuses can be located in several different locations depending on the make and model of the microwave. To locate a fuse, refer to the microwave’s wiring diagram or manual.

To check the fuses:

1. Disconnect the power to the microwave.
2. Remove the microwave cabinet.
3. Remove the control panel (if necessary—depends on the microwave model).
4. Unscrew or unclip the fuse to remove it from the microwave.
5. Test the fuse with a multimeter for continuity.
6. If defective, replace the fuse with a new one that matches the fuse you removed. Installing the wrong fuse will cause it to blow again or to fail to work correctly.

3. Defective Diode

The microwave’s diode converts alternating current (AC) to direct current (DC), which doubles the voltage and powers the magnetron that heats the food. If the diode is defective, the magnetron will not receive enough voltage, and the microwave will not be able to heat the food.

To determine if the diode has failed, the diode can be examined for signs of damage. It can also be tested with a multimeter and a 9-volt battery to check if the diode has continuity. The diode should only show continuity in one direction. If there is no continuity in both directions or there is continuity in both directions when the leads are reversed, the diode has failed and will need to be replaced.

To access and check the diode:

1. Disconnect the power to the microwave.
2. Remove the microwave cabinet.
3. Discharge the capacitor (see safety warning above).
4. Remove the diode from the microwave.
5. Inspect the diode for signs of damage.
6. Test the diode with a multimeter and a 9-volt battery for continuity.
7. Replace a faulty diode.

4. Defective Magnetron

The magnetron is responsible for creating the microwave frequencies that heat the food. If the magnetron is failing or defective, the microwave will not heat properly. Other symptoms of a defective magnetron include food not cooking properly and an increase in the typical hum a microwave makes when cooking.

A multimeter can be used to test the magnetron for continuity. The multimeter readings should be less than 1 ohm for the magnetron to work correctly. If the magnetron is defective, it will need to be replaced.

To access and check the magnetron:

1. Disconnect the power to the microwave.
2. Remove the microwave cabinet.
3. Discharge the capacitor (see safety warning above).
4. Remove the magnetron and test it for continuity with a multimeter.
5. Replace a defective magnetron.

5. Defective High-Voltage Capacitor

The high-voltage capacitor works with the diode to convert the alternating current to direct current and double the voltage. If the capacitor is defective, the microwave will not be able to get the voltage it needs to heat and cook food.

A VOM meter with a capacitance testing capability can be used to determine if the high-voltage capacitor has failed. However, remember the safety warning. The capacitor will need to be discharged to avoid injury.

To access and check the high-voltage capacitor:

1. Disconnect the power to the microwave.
2. Remove the microwave cabinet.
3. Discharge the capacitor (see safety warning above).
4. Remove the capacitor.
5. Check the capacitor with a VOM meter that allows for capacitance testing.
6. Replace a faulty high-voltage capacitor.

6. Failed High-Voltage Transformer

Another reason the microwave may not be heating is that the high-voltage transformer has failed. A defective transformer will often arc (a rapid release of energy) and produce a burning smell. The transformer has three windings, which can be tested with a multimeter to determine if the transformer needs to be replaced.

How to access and check the high-voltage transformer:

1. Disconnect the power to the microwave.
2. Remove the microwave cabinet.
3. Discharge the capacitor (see safety warning above).
4. Remove the transformer from the microwave.
5. Test the transformer with a multimeter.
6. Replace a defective transformer.

7. Defective Main Control Board

Faults with the main control board are uncommon; however, if the above components are working correctly, the main control board may have a fault. The control board can be checked visually for signs of burning or a short circuit. If the main control board is defective, it will need to be replaced.

To access and check the main control board:

1. Disconnect the power to the microwave.
2. Remove the microwave cabinet.
3. Remove the main control board.
4. Check the control board for signs of burning, arcing, or a broken connection.
5. Replace a defective control board.