How to Repair a Microwave Oven Not Heating

The compact and convenient microwave oven is excellent for reheating leftovers, warming hot beverages, defrosting, and more. If a microwave is no longer heating as it should, and you want to attempt to repair it before purchasing a new one, this article is for you.

We will go through the different problems that may be causing a microwave not to heat, offering step-by-step instructions to fix it.

Important: Safety first

Before we start, it’s essential to understand that repairing a microwave is dangerous and should only be done by professionals. The microwave must be unplugged and the high-voltage capacitor discharged before attempting any of the fixes in this article.

The capacitor can hold a lethal charge even when the microwave is unplugged, so it must be discharged to avoid potentially fatal electrocution when carrying out repairs.

Here’s how to discharge the capacitor:

1. Unplug the microwave.
2. Put on a pair of rubber gloves.
3. Unscrew and remove the back and side panels on your microwave.
4. Place a screwdriver or plyers with an insulated handle across each set of the capacitor’s terminals and wait for a few seconds. This will cause the capacitor to short, discharging any stored energy.
5. Be careful you do not touch the metal part of the tool when doing this.

How to fix a microwave oven that isn’t heating

It’s time to get started with the repairs. Here are the five top reasons your microwave is not heating and how to fix it.

1. Defective door switch

For safety reasons, a microwave will not heat if the door is open. If one of the door switches on your microwave is faulty, it may send the signal that the door is open, even when it isn’t, which prevents the microwave from heating. 

There are typically three or four door switches on a microwave. You will need to test each switch with a multimeter for continuity to see if one has failed. Here’s how:

1. Unplug your microwave.
2. Remove the microwave’s outer cover.
3. Discharge the capacitor.
4. Remove the screws securing the control panel.
5. Take a picture of the wiring on the control panel. Then disconnect the wires so you can remove the control panel.
6. Take a picture of the wire connections on the door switches. You may need to unscrew the door switch assembly mounting bracket first for better access.
7. Disconnect wires to the door switches.
8. Test each door switch for continuity with a multimeter.
9. Replace any door switches that don’t have continuity.
10. Reconnect the wires according to your pictures and reassemble the microwave.

2. Faulty magnetron

The magnetron generates and emits the microwave radiation needed to heat your food. If the magnetron is broken or burned out, the microwave will still run but won’t heat. Once broken, the magnetron cannot be repaired—only replaced.

Replacing the magnetron is not cheap, so you will need to decide whether it’s worth replacing or if you should just buy a new microwave. Here’s how to test the magnetron:

1. Unplug your microwave.
2. Remove the microwave’s outer cover.
3. Discharge the capacitor.
4. Remove the screws securing the magnetron and disconnect the wires connected to it. The magnetron is usually located in the center of the microwave.
5. Test the magnetron with a multimeter for continuity.
6. If the magnetron doesn’t have continuity, it will need to be replaced.

3. Burned-out high-voltage diode

The high-voltage diode helps generate the high voltage needed to power the magnetron. If the diode burns out, your microwave will still run but won’t heat. Here’s how to check the high-voltage diode:

1. Unplug your microwave.
2. Remove the microwave’s outer cover.
3. Discharge the capacitor.
4. Locate the high-voltage diode. It’s normally connected to the magnetron and capacitor.
5. Remove the diode by disconnecting it from its terminals.
6. Inspect the diode for visible signs of damage.
7. Set a multimeter diode test mode. Place the multimeter’s red probe on the high-voltage diode’s anode and the black probe on its cathode. The reading should be between 0.3V and 0.7V.
8. Next, place the multimeter’s red probe on the high-voltage diode’s cathode and the black probe on its anode. The reading should be OL.
9. If the diode fails either of these tests or looks damaged, you should replace it.

4. Defective high-voltage capacitor

The capacitor works with the high-voltage diode to convert AC (alternating current) to DC (direct current) and increase the voltage to power the magnetron. If the capacitor is faulty, the microwave won’t be able to heat.

Here’s how to check the high-voltage capacitor:

1. Unplug your microwave.
2. Remove the microwave’s outer cover.
3. Discharge the capacitor.
4. Disconnect the wires connected to the capacitor and remove any screws securing it in place.
5. Test the capacitor with a multimeter set to capacitance mode.
6. Place the multimeter probes on the capacitor terminals, ensuring the red probe is on the positive terminal and the black probe is on the negative terminal.
7. Compare the reading to the capacitance value printed on the capacitor. If the reading deviates significantly or shows zero or an infinite reading, it will need replacing.

5. Failed high-voltage transformer

The high-voltage transformer converts power from standard voltage to the high voltage needed to power the magnetron. When a transformer fails, it will often produce a burning smell. The microwave will also no longer heat properly.

Here’s how to test whether the high-voltage transformer has failed:

1. Unplug your microwave.
2. Remove the microwave’s outer cover.
3. Discharge the capacitor.
4. Locate the transformer. It’s a large, rectangular component usually found at the bottom of the appliance.
5. Inspect the transformer for signs of damage, charring, or a burnt smell.
6. Disconnect the wires connected to the transformer and remove it from the microwave.
7. Test the primary winding, low-voltage winding, and high-voltage winding one at a time with a multimeter set to ohms.
8. The reading should be five ohms or less for the primary winding, one ohm or less for the low-voltage winding, and between 35 and 150 ohms for the high-voltage winding.
9. If the transformer fails any of these tests, it should be replaced.