Microwave won’t heat: 10 common problems & solutions

Samantha Bullington / Thursday February 8, 2024

Ever stood in front of your microwave, tapping your foot impatiently, only to realize your meal is as cold as when you put it in? You’re not alone!

Before you start mourning the loss of this kitchen companion or dialing up the nearest appliance repair service, hold on! We’ve got some troubleshooting tips that might just bring your microwave back to life.

If you need any replacement parts for your microwave, 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 videos to show you how to install your new parts.

Contents hide

I Microwave won’t heat: 10 common problems & solutions

I.I Before you start troubleshooting

I.I.I How to discharge a microwave capacitor

I.I.II More safety precautions

I.II 10 common reasons why a microwave won’t heat

I.II.I 1. Waveguide or cavity damage

I.II.II 2. Faulty door switches

I.II.III 3. Blown thermal fuse

I.II.IV 4. Defective diode

I.II.V 5. Bad turntable motor

I.II.VI 6. Faulty cooling fan

I.II.VII 7. Defective magnetron

I.II.VIII 8. Bad transformer

I.II.IX 9. Faulty high-voltage capacitor

I.II.X 10. Defective control board

I.III More information

I.III.I Share this entry

Before you start troubleshooting

Please be very careful while working on a microwave since it contains high-voltage parts.

Always unplug the microwave and discharge the high-voltage capacitor before working on it to eliminate the risk of electric shock.
With the microwave unplugged, remove the outer case to gain internal access.
Never power on the microwave without the cover in place.
Wear insulating gloves and use insulated tools when testing any parts.
Check all parts for damage before reassembling and powering up the microwave again.

How to discharge a microwave 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 do not feel comfortable discharging a capacitor, do not attempt to do it yourself.

Unplug the microwave from the power supply.
Grab a screwdriver with good insulation on the handle to protect against electric shock.
Gently touch the insulated screwdriver blade to one terminal of the capacitor.
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.
Repeat the process to create a short circuit between each capacitor terminal and the chassis (bare metal mounting plate surface). If the capacitor has three terminals, use the same method.

More safety precautions

Microwaves contain high-voltage parts and can 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 unplug the microwave before troubleshooting or making repairs.
Discharge the high-voltage capacitor before working on high-voltage parts to eliminate the risk of electric shock.
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.
With the microwave unplugged, remove the outer case to gain internal access.
Wear insulating gloves and use insulated tools when testing any parts.
Check all parts for damage before reassembling and powering up the microwave again.
If the microwave 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 clearly see and access microwave 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 microwave or replacement part.
Be gentle when handling or removing parts. Excessive force might damage the microwave 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.
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.

10 common reasons why a microwave won’t heat

READ MORE: How to use a multimeter

1. Waveguide or cavity damage

If the waveguide – which channels microwaves from the magnetron – or the interior cavity is damaged, either issue can prevent the microwave from heating.

Cracks, dents, or holes in the waveguide can cause microwave leaks. If the food cavity has become distorted, it may prevent microwave energy from being distributed.

Check the waveguide and the cavity for damage. If you find any, replace them as needed.

2. Faulty door switches

Door switches are some of the most frequently faulty parts of a microwave. Located along the door frame, they’re designed to cut power to the microwave when the door is open.

Most microwaves include three switches located behind the control panel: primary, secondary, and monitor switches, which all actuate to prevent the microwave from cooking while the door is open.

You can test both the primary (top) and secondary (bottom) switches for continuity:

Make sure the microwave is turned off and disconnected from the power supply.
Open the door to test the primary (top) and secondary (bottom) switches.
Keep the door open and place the two multimeter leads on the switch connectors.
When you close the door, the multimeter should produce a beep to signal continuity if the switch is good. If you don’t hear a beep or get an incorrect reading (refer to your manual for the acceptable range), replace the switch(es).
Test the monitor (middle) switch with the door closed. Place the two multimeter leads on the monitor switch connectors.
When you open the door, the meter should beep to signal continuity. If you don’t hear a beep or get an incorrect reading (refer to your manual for the acceptable range), replace the switch.

3. Blown thermal fuse

Microwaves contain several thermal fuses and thermal protectors. These safety components are designed to blow and cut power if the oven overheats.

Check the thermal fuses and thermal protectors for continuity by using the two leads of a multimeter. If you don’t get continuity, then the fuse has blown and should be replaced.

4. Defective diode

The high-voltage diode works together with the capacitor to convert incoming alternating current (AC) to direct current (DC) and doubles the voltage sent to the magnetron tube. If the diode burns out, the magnetron won’t receive enough voltage to power on and heat.

A bad diode will often appear burnt or cracked. Here is how to test it:

Make sure the microwave is unplugged and the high-voltage capacitor is discharged.
Remove the diode from the circuit.
Use a 9V battery as a power source.
Use a multimeter to check diode voltage in both directions. You can do this by placing one diode terminal onto one of the battery posts. Then use the meter probes to test from the other terminal end to the open battery post. Reverse the diode terminals you used and test for voltage again. There should be 9V shown on only one terminal when testing from the diode terminal to the open battery post.

A working diode will show voltage in only one electrical direction. If the diode shows voltage in both directions, it’s defective and needs to be replaced.

5. Bad turntable motor

If the turntable stops turning, it can mess up the microwave’s heating performance. Food needs to rotate consistently inside the microwave to heat up evenly.

Take a look at the central hub, support rollers, and underside of the turntable for any damage. Replace it if you find any obvious damage.
Test the motor for resistance with a multimeter. Set the meter to Ohms and place the meter leads on the motor terminals. You should get a reading of approximately 2-4 ohms (check the tech sheet for your unit for specifications). If you don’t get a good reading, replace the motor.

6. Faulty cooling fan

Microwaves require internal air circulation and ventilation to prevent overheating. A defective cooling fan or a faulty exhaust fan motor can cause buzzing, rattling, or grinding noises while the microwave is running.

How to check it:

Clear any obstructions from all the vents.
Find the exhaust fan motor and other cooling fan motors.
Remove each fan and try spinning the blades with your fingers. If the blades don’t spin freely or if they scrape loudly, then the fan needs to be replaced.

7. Defective magnetron

The magnetron is the heart of the microwave—it’s what converts electrical energy into microwave energy. If the magnetron burns out, your oven may still turn on, but it won’t heat the food.

How to check it:

Unplug the microwave and discharge the high-voltage capacitor.
Remove the outer panel to access the magnetron.
Set your multimeter to test resistance in Ohms (the symbol looks like an upside-down horseshoe).
Touch the meter probes to the magnetron’s contacts.
Check the reading on the multimeter display. A magnetron should show a very low Ohm reading, usually less than 1 Ohm of resistance. Higher resistance or no resistance means you have a defective magnetron that needs to be replaced.

8. Bad transformer

The transformer is responsible for stepping up the standard household voltage to the 2000V+ needed to operate the magnetron. If it fails, the microwave won’t have sufficient power to turn on and heat up.

Signs of a faulty transformer include a burning electrical smell, arcing noises, or physical damage like melted wires or cracks in the casing. If the transformer is burnt out, you’ll need to replace it.

9. Faulty high-voltage capacitor

The high-voltage capacitor works hand-in-hand with the diode rectifier. It stores microwave energy and helps consistently power the magnetron. If the capacitor fails, the microwave won’t have enough power to heat properly.

How to test the capacitor for resistance with a multimeter:

Make sure the microwave is off and unplugged.
Make sure the capacitor is discharged.
Set your multimeter to resistance.
Touch the meter leads to the capacitor terminals. You should see a resistance reading on the meter and it should drop as you hold the leads. If it doesn’t, then you’ll need to replace the capacitor.

10. Defective control board

The control board regulates all the microwave components and circuits. It can fail over time and prevent the oven from heating. A faulty control board is rare, and usually isn’t the culprit behind a non-heating microwave.

Be sure to test all other parts like the diode, magnetron, and capacitor first. If those components check out okay, then the control board may be defective. Replacing it with a known good board can get your microwave fully functional again.

READ MORE: 8 DIY Safety Tips

More information

Thanks for reading! We hope this guide helped you repair a microwave that isn’t heating.

If you have any other appliance repair needs or projects, you can find more resources including our DIY blog at AppliancePartsPros.com. There, you can enter your model number to order the exact parts you need. Most orders arrive in two business days.

You can also reach our award-winning customer service team at 1-877-477-7278, chat with a pro on our website and watch thousands of free video tutorials on our YouTube channel. We can also help you troubleshoot common microwave problems here.

Make sure to follow us on Facebook, Twitter and Instagram!

Share on Facebook

Save

Samantha Bullington

Hey there, I’m Samantha and I’m super excited to be a part of the Appliance Parts Pros team! With 9 years of experience, I’ve had the awesome opportunity to assist countless customers in troubleshooting their home appliances and finding DIY repair solutions.