Microwave turning on by itself: A troubleshooting guide

It’s the middle of the night and you hear your microwave turn on. No buttons were pressed and no timers were set, yet it started heating all by itself.

Is your appliance possessed? It’s unlikely. Regardless, a microwave that turns on by itself – particularly if you have items inside – can pose a serious fire risk and threaten your safety.

So what’s going on? There are a few things we can check and test to try to isolate the problem and help you sleep better at night.

Disclaimer: The troubleshooting information provided in this article is based on general service manuals and 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.

Safety tips

You need to be very careful when testing microwaves since they contain high-voltage components. Here are some guidelines to prevent yourself from getting zapped:

• Always unplug the unit and discharge the high-voltage capacitor before disassembling the microwave to prevent electric shock.
• With the microwave unplugged, remove the outer case to gain internal access.
• Never power on the microwave without the cover in place, as this protects it from radiation.
• Wear insulating gloves and use insulated tools when testing components.
• Check all parts for damage before reassembling and powering up again.

Read More: How to use a multimeter

How to discharge a capacitor 

IMPORTANT: Make sure to carefully follow the correct handling and discharge procedures since exposure to the voltage levels inside a microwave can potentially cause severe injury or death.

1. With your microwave unit unplugged, touch the blade of a well-insulated screwdriver to one terminal. 
2. Gently slide the screwdriver forward until it reaches the other terminal and hold it there for a few seconds. Be aware that this often results in a loud and startling ‘POP’.
3. Repeat this procedure to create a short circuit between each capacitor terminal and the chassis (bare metal mounting plate surface). This same method can be applied to a capacitor having three, and not two, terminals.

Read More: How to clean a microwave

Troubleshooting steps 

Now that you’re familiar with the major hazards involved, we can walk through some troubleshooting steps to find out why your microwave is turning on by itself:

Step 1 – Check the exterior

Examine the outside of the microwave for any damages or abnormalities. Make sure to look at all sides, vents, the rear panel, and the top. 

If there are any burning smells coming from the unit during normal operation, that signifies there are overheating components that will likely fail soon.

Damage in these areas may provide clues as to why the microwave is turning on by itself and can cause short-circuiting:

• Door
• Hinges
• Chassis
• Touchpad (especially if it’s melted)

Read more: Painting the Inside of a Microwave

Step 2 – Test the door switches

One of the most common reasons why a microwave may turn on by itself is stuck switches. Most microwaves include three switches located behind the control panel: primary, secondary, and monitor switches, which all actuate to keep the microwave from cooking while the door is open.

How to test the primary (top) and secondary (bottom) switches:

1. Test the primary (top) and secondary (bottom) switches with the door open. 
2. Keep the door open and place the multimeter leads on the connectors on the switch. 
3. When closing the door, the multimeter should produce a beep if the switch is good.

How to test the monitor switches:

1. Test the monitor (middle) switch with the door closed. 
2. When you open the door, the meter should beep.

If you did not hear a beep when testing any or all three of these switches, replace the switch.

Step 3 – Check the touchpad for short-circuiting

The touchpad and control panel on the microwave door that houses the buttons is another area susceptible to shorts. Touchpads can crack or buttons can stick due to wear over time. If the membrane circuits responsible for sensing button presses on the touchpad are damaged or intermittently making contact, it may register random input commands and cause the microwave to turn on by itself.

The random signals can trigger preprogrammed cook times and power levels that could turn on the magnetron and start heating. If the short is more severe, it may even engage the highest power setting, resulting in an unintended yet full burst of microwave energy.

If the main control board is good, check for issues with the touchpad panel containing all the buttons. Warning signs of a faulty touchpad:

• The unit activates when pressing certain buttons, indicating a short. (This does not include the quick start buttons)
• Buttons work sometimes but not always, which suggests a loose connection.
• One or multiple buttons don’t react at all. This means they have open circuits.

Test the buttons

Press and release each button on the touchpad and control panel. Do this several times for each one. See if the correct touch tones sound and display illuminates while pushing each button.

You can also try pressing multiple buttons in a sequence to see if specific combinations inadvertently start the microwave. This will help narrow down possible touchpad short locations.

• Any buttons that aren’t responding likely have faulty contacts.
• If pressing one button (other than quick start buttons) activates the unit, it means there’s a short.

Test the touchpad

1. Visually inspect the touchpad for any damaged, burned, or unresponsive buttons. Replace the entire pad if issues are found. Touchpads are typically low-cost repair parts.
2. Use a multimeter set to ohms and check for continuity (closed circuits) when buttons are pressed. If certain buttons have resistance or no continuity, it signifies a faulty membrane switch, which means the touchpad needs to be replaced.
3. Look for cuts, breaks, or discoloration on the conductive traces underneath the pad material.
4. Check for loose, damaged, or corroded connections where the pad plugs into the main control board.
5. Test full button functionality like time setting, power levels, and start/stop.
6. If you can, temporarily remove the touchpad connections and use a known good pad to narrow down the fault(s).

Read More: How to replace a microwave light bulb

Step 4 – Check the main control board

The main control board is the central component that controls the various electrical operations of the microwave. This board houses multiple circuits and relays that engage the microwave’s cooking, lighting, turntable, safety interlocks, and other features.

Some signs of control board problems include:

• Odors emitting from the unit that suggest the board is overheating.
• Scorched or burnt spots on the board, which point to a short.
• Error code displays related to board failures.
• A non-functioning unit.

If one or more of the soldered connections or circuits on the main control board have shorted out, it can cause errant signals that activate cooking functions when no buttons are pressed. A short circuit on the board that overrides the door safety interlocks can also cause the microwave to turn on while the door is open.

If any signs of short-circuiting are confirmed, the main control board will likely need to be replaced.

Warning: Unless you’re experienced with appliance electronics, visually inspecting the control board is the safest option. Check with the microwave’s manufacturer support for any specific component value verifications for the board. 

How to inspect and test the board:

1. Visually check the board for burnt or bulging components or signs of physical damage. Replace it if anything looks abnormal.
2. With a schematic diagram, use a multimeter to ohm out any low-voltage control circuits for open/shorted traces.
3. Attempt to power on the microwave while watching for obvious faults like smoke, bad smells, or failure of low voltage circuits.
4. As a last resort, you can temporarily swap in a new known good board to confirm that the old board is faulty.

Read More: How to replace a microwave turntable coupler

More information

Thanks for reading! We hope this information helped you determine why your microwave is turning on by itself. 

If you have any other appliance repair needs or projects, you can find more resources and our DIY blog at AppliancePartsPros.com. There, you can enter your model number to order the exact parts you need. Most orders arrive in 2-3 business days.

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