How to troubleshoot a microwave that keeps blowing fuses

If you’re dealing with a microwave that keeps blowing fuses, don’t trash it before trying some DIY troubleshooting tricks. We’re here to help you get your unit back up and running.

If you find out that you need any replacement parts for your microwave while you’re troubleshooting, you can enter your model number at AppliancePartsPros.com to order them. Most orders arrive in just two business days, and we have thousands of free guides to show you how to install your new parts.

Important: The troubleshooting information provided in this article may not apply to your specific appliance model. While we strive to offer helpful advice for DIY appliance repairs, variations in models and configurations can lead to differences in troubleshooting steps, error codes and part replacements. For the most accurate guidance, we strongly recommend consulting the technical sheet for your specific appliance model.

Read more: How to Install and Replace a Microwave

Safety precautions

Microwaves contain high-voltage parts and have the capacity to store voltage even when they’re disconnected from a power source. Please be very careful while working on a microwave’s internal parts to avoid electrocution, which has the potential to cause serious injuries or death.

• Always power off and unplug your appliance or switch off the circuit breaker before you attempt any maintenance or replacement work. This keeps you safe by eliminating any risk of electric shock.
• Discharge the high-voltage capacitor before working on high-voltage parts to eliminate the risk of electric shock. Don’t continue if you are uncomfortable or unfamiliar with this process.
• Never attempt to run the microwave while its casing is open or when internal parts are exposed. Always completely reassemble the microwave before testing or using it.
• Never attempt to bypass any microwave safety features.
• Never run the microwave empty. Use a glass of water or a bowl of food for testing.
• Removing a microwave from over the range is a two-person job due to the weight and potential for damage. Always follow manufacturer installation instructions to reinstall the unit over the range after repairs have been made.
• If the appliance has recently been used, give it plenty of time to cool down before working on it.
• Take your time while working to prevent accidents and personal injuries. Rushing is the enemy of precision.
• Work in a well-lit area so you can see and access appliance parts.
• Keep your workspace free of clutter and other obstacles. Keep children and pets away from the work area.
• Never work on internal parts with wet hands. Make sure the work area is completely dry.
• Check the user manual to see if there are specific installation or safety instructions related to your appliance or replacement part.
• Be gentle when handling or removing parts. Excessive force might damage the appliance or cause personal injury.
• Wear insulated work gloves to protect your hands from sharp metal parts and debris.
• When working with wires, avoid touching any exposed wires or terminals. If you need to touch a wire, use a non-conductive tool or wear insulating gloves to prevent electrical shock.
• Always take photos or make a note of wiring terminals or other connections before disconnecting them to make reassembly easier.
• Don’t test live voltage with a multimeter if you’re unfamiliar with how to prevent short-circuiting.
• Consider wearing safety glasses and/or a dust mask when working with chemicals, dust or a large amount of debris to prevent irritation or injury.

Read more: How to Clean a Microwave  

How to troubleshoot your microwave

Follow these steps to fix a microwave that keeps blowing fuses.

Step 1: Discharge the microwave’s capacitor 

IMPORTANT: Carefully follow the correct handling and discharge procedures since exposure to the voltage levels inside a microwave can potentially cause severe injury or death. Prioritize safety at all times and follow the instructions and recommendations provided by the manufacturer. If you don’t feel comfortable discharging a capacitor, don’t attempt to do it yourself.

1. Unplug the microwave from the power source.
2. Grab a screwdriver with good insulation on the handle to protect against electric shock.
3. Gently touch the insulated screwdriver blade to one terminal of the capacitor.
4. Slowly and gently slide the screwdriver forward until it reaches the other terminal of the capacitor. Hold it there for a few seconds. This discharge process often results in a loud and startling ‘POP’ sound.
5. Repeat the process to create a short circuit between each capacitor terminal. If the capacitor has three terminals, use the same method.

Step 2: Check the power supply

LIVE VOLTAGE WARNING: Please be very careful to protect yourself from electric shock, which has the potential to cause serious injuries. Don’t ever test live voltage if you’re uncomfortable using a multimeter.

1. Examine the power outlet. Look for any burn marks, debris or other damage. If the outlet is damaged, you’ll need to contact a licensed electrician to fix it.
2. Set a multimeter to voltage AC mode.
3. Use the meter probes to test the voltage at the power outlet. The voltage should be within the microwave’s operating range, which you can find in the unit’s user manual.

If you get an unacceptable reading from the power outlet, contact a licensed electrician to fix the problem.

Step 3: Check the door lock switches

1. Make sure the unit is disconnected from the power source.
2. Find the door lock switches, which are usually near the hinges or latch mechanism.
3. Set a multimeter to continuity mode.
4. Use the multimeter probes to test the switches for continuity. With the door open, the primary and secondary switches shouldn’t have continuity. With the door closed, the switches should have continuity. The middle monitor switch will have the opposite results.

If the switches give incorrect readings, then they’re faulty and should be replaced.

Step 4: Check for shorted wiring

1. Inspect the wiring for any signs of damage, such as fraying, melting, or shorting.
2. Use a multimeter to test the resistance of the wiring. The readings should be within the manufacturer’s specifications, which you can find on the technical sheet for your model.

If any of the wiring is damaged or shorted, replace it.

Step 5: Test the capacitor and diode

Important: Working with high-voltage components in a microwave can be extremely dangerous. If you’re not confident in your ability to perform these tests safely, it’s best to consult a qualified appliance repair technician.

How to test the diode:

1. Find the diode in the microwave’s circuit. It’s typically small and cylindrical with two leads.
2. Set your multimeter to the diode test function or the “Rx1” setting.
3. Touch the positive (red) probe of the multimeter to the anode (usually the larger lead) of the diode and touch the negative (black) probe to the cathode (usually the smaller lead).
4. Check the reading. The multimeter should display a reading indicating that the diode is forward-biased.
5. Reverse the probe positions and touch the positive probe to the cathode and the negative probe to the anode. The multimeter should display a low or no reading.

If the diode does not show the expected readings, then it’s faulty and needs to be replaced.

How to test the capacitor:

1. Find the capacitor in the microwave’s circuit. It’s typically a cylindrical or rectangular shape with two leads.
2. Set your multimeter to the capacitance measurement function.
3. Touch the positive (red) probe of the multimeter to one lead of the capacitor, and the negative (black) probe to the other lead.
4. Check the meter reading. If the capacitor is reading 0 or significantly different from the expected value, it’s faulty and needs to be replaced.

Step 6: Test the high-voltage parts

Important: Working with high-voltage components in a microwave can be extremely dangerous. If you aren’t confident in your ability to perform these tests safely, it’s best to consult a qualified appliance repair technician. 

How to test the high-voltage transformer:

1. Find the high-voltage transformer, which is typically the large, rectangular part near the magnetron.
2. Set your multimeter to the resistance function and refer to the manufacturer’s technical information before moving forward with testing.
3. Touch the positive (red) meter probe to one primary terminal of the high-voltage transformer and the negative (black) probe to the other primary terminal. Then test the secondary terminals in the same manner.
4. Check the meter reading. The meter should display a reading within the manufacturer’s specified range.

If the reading is significantly lower or higher than expected, then the transformer is faulty and needs to be replaced.

How to test the magnetron:

1. Find the magnetron, which is typically the large, cylindrical part near the high-voltage transformer and capacitor.
2. Set your multimeter to the resistance (Ω) function.
3. Touch the positive (red) meter probe to one terminal of the magnetron and the negative (black) meter probe to the other terminal.
4. Check the meter reading. The meter should display a reading of a few Ohms, indicating that the magnetron’s filament is intact.

If the reading is significantly different from the expected value or the magnetron isn’t heating up while the microwave is running, then it’s faulty and needs to be replaced.

Read more: 10 reasons why a microwave won’t heat  

Additional information

Thank you for reading! We hope this guide helped you get your microwave working again.

If you have any other appliance repair needs or projects, don’t hesitate to explore our other troubleshooting guides and videos.

If you need replacement parts or some extra help with ordering them, grab your model number and head over to AppliancePartsPros.com, where you can chat with a pro, read our DIY blog and more.

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