If your microwave has power but is not heating or cooking food, there are a few common issues that could be causing the problem. In this article, we will walk through some quick troubleshooting steps and explain the most likely culprits when a microwave has power but won’t work.
Check That the Door is Closing Properly
One of the simplest explanations for why a microwave has power but won’t heat is that the door is not closing properly. Microwaves have safety interlocks that prevent the microwave from operating when the door is open. This prevents harmful microwave radiation from escaping.
To check the door, first make sure it is clean with no food particles stuck that could be obstructing proper closure. Try closing it normally and see if it seals fully. Sometimes the latch can become slightly out of alignment if the door has been slammed too hard. Try gently realigning the latch or applying a bit more pressure when closing to get a good seal.
If the door still seems to be the issue, inspect the door hinges and hooks to make sure there are no breaks or malfunctions. Make sure the door switches (small buttons that get pressed when the door closes) are also clean and not stuck. The switches form part of the safety interlock system.
Test the Power Supply
Assuming the door is closing properly, the next thing to check is the power supply. A microwave requires 120V power at 60Hz to operate correctly. Here are some things to test:
- Make sure the microwave is plugged into a working outlet and not connected to a tripped breaker, damaged cord, or faulty GFCI outlet.
- Test the outlet with another appliance like a lamp to verify it has electricity.
- Inspect the power cord for damage and make sure it is firmly plugged into the microwave.
- Use a multimeter to check for 120V at the outlet while the microwave is plugged in. Lack of correct voltage can prevent heating.
If the microwave is getting 120V power but still won’t heat, the issue likely lies in an electrical component failure inside the microwave.
Test the Magnetron
The magnetron is the microwave’s source of microwaves for generating heat. It is a tube that works similarly to the cathode ray tube in old TVs. If the magnetron burns out, the microwave won’t be able to produce any heating microwaves.
You can test if the magnetron is working using a multimeter. To access the magnetron, you will need to remove the outer cabinet. Once exposed, set your multimeter to test resistance or continuity. Place one probe on the magnetron chassis and one probe on one of the magnetron antenna leads. A good magnetron should show very low resistance, usually between 1-10 ohms.
If you get a significantly higher reading, it indicates the magnetron filament is burnt out and needs replacement. Never power on a microwave when the case is removed for safety reasons.
When a new magnetron is required, the specific replacement part number can usually be found on the original magnetron’s label. Search for this part number online to find a matching replacement magnetron. Installation is typically a matter of disconnecting the old tube, removing mounting fasteners, and reversing the process with the new magnetron.
Replacing a magnetron yourself can save on repair costs but does carry electrical risks if proper precautions are not taken.
Microwave Not Heating but Fan and Lights Work
In some cases when the microwave won’t heat, you may notice the interior fan and lights still turn on. This points to an issue with the high voltage diode. This crucial component converts incoming 120V AC power into the high-voltage DC needed for magnetron operation. When it fails, the low-voltage systems still work but the high-voltage magnetron circuit is disabled.
You can test a suspicious high voltage diode with a multimeter just like testing the magnetron. A good diode will show low resistance in one direction and very high resistance when polarity is reversed. If your readings indicate the diode is faulty, a replacement matched to your model is needed.
Microwave Display is Blank
If the microwave’s display is completely blank and no interior lights or fans activate when the door is closed, there is likely a problem with the keypad and control board. The control board regulates all the microwave’s functions based on input from the keypad.
Try unplugging the microwave and plugging it back in to reset the controls. If the display remains blank, remove the outer chassis and inspect the ribbon cables and wiring connecting the keypad to the small control board. Reflow any loose solder joints and make sure no wires or connectors are damaged.
If that fails to restore operation, the control board may need professional troubleshooting or replacement. The boards contain delicate controllers and circuity that are difficult to properly test without specialized equipment.
Microwave Display Works But Won’t Start
In some cases, the display will illuminate and show programming options but hitting Start does not activate the microwave. This can be due to a faulty door interlock switch. Although the switches allow the display to function, they interrupt the high power circuit as a safety precaution when triggered.
Try adjusting the door alignment and latching forcefully to ensure the switches are being firmly pressed when closed. If the problem persists, the switches themselves would likely need replacement.
Displaying but not starting can also happen when the thermal cut-out has tripped. This is a one-time fuse that blow if the microwave significantly overheats due to failure of components like the magnetron. If tripped, the thermal cut-out would need replacement by a service tech for the microwave to operate again.
Microwave Runs but Produces No Heat
In the case of a microwave that appears to run with the turntable spinning normally but not actually heating food, the most likely issue is a faulty diode or magnetron. As described previously, problems with either of these components can disable the microwaves but allow lower voltage functions to still operate.
Testing the diode and magnetron and replacing any faulty parts is typically how this issue would be addressed. In rare cases, there could be damage to the microwave waveguide that channels emitted waves – inspection would reveal any physical damage present.
Microwave Not Heating Evenly
If some parts of food seem to heat more than others in your malfunctioning microwave, uneven distribution of internal microwaves is likely to blame. This can happen for a few reasons:
- Magnetron is going bad and producing weaker microwaves
- Blocked or damaged waveguide is causing hot/cold spots
- Turntable motor is not smoothly rotating
- Microwave cavity has sustained damage
Testing the magnetron’s output and condition of the waveguide is important. The turntable motor can be checked for smooth spinning when running. Lastly, physical inspection of the internal cavity may reveal dents, cracks, or arcing damage causing poor microwave reflections.
Sparking Inside Microwave Oven
Seeing electric arcing or sparking inside your microwave is very dangerous and means it requires immediate repair. This is most often caused by damage to internal microwave components that is allowing electricity to arc to the surrounding metal cavity.
The main causes of sparking inside a running microwave are:
- Faulty or cracked magnetron allowing high voltage leakage
- Damaged or corroded waveguide
- Microwave cavity sustaining cracks or holes
- Door seal or chassis leaks allowing microwaves to arc outside chamber
Any sparking should prompt an immediate inspection of the above components. Often the faulty part will show signs of damage or corrosion accompanying any arcing marks on the cavity walls. The microwave is unsafe to operate until the cause of sparking is repaired.
Microwave Keypad Malfunctioning
If your microwave keypad is unresponsive or only working intermittently, ribbon cable damage or a problem with the control board could be the issue. Try power cycling the microwave and check if the keypad begins working again. Look for any loose or worn cables going from the keypad buttons to the small control board inside the unit.
If the keypad seems intact, the control board itself likely has an issue and would need professional service. The control board regulates power distribution and input response based on the user keypad. Damaged circuits or capacitors on the boards can prevent normal operation.
In rare cases, the issue could be a bad fuse on the control board that needs replacement by de-soldering the old fuse and installing a new one of the same amperage.
Unusual Noise Coming from Microwave
Some occasional humming or low sounds are normal from microwaves, but loud banging, grinding, or buzzing noises likely indicate a problem needing repair. Here are some possible causes of unusual microwave noises:
- Bad door interlock allowing microwave operation with door open
- Damaged fan blade scraping the interior
- Bad fan motor bearings
- Faulty high voltage components arcing
- Bad diode and magnetron causing interference
Check the door thoroughly for proper closure and interlocks if the noise only happens with the door open. If it occurs all the time, carefully inspect the interior fan, magnetron, waveguide, and cabling for anything that could be causing audible vibration or arcing. Unusual microwave noises signify potentially dangerous conditions for components and users.
Microwave Will Not Turn On At All
If none of the previous troubleshooting steps restore operation to your microwave, there are a few other things that can cause a complete malfunction:
- No power to unit – test outlet, cord, and breaker
- Burnt out interior fuse needing replacement
- Shorted door interlock switches disabling all function
- Bad main control board which may require pro service/replacement
Use a multimeter to check for power at the outlet and test continuity of the internal fuses. Inspect door interlock switches for shorts and try manually activating them with the door open to test for function. If those components all check out, the main control board is likely the issue in cases of a “dead” microwave.
Microwaves are complex appliances with many components working together to safely heat and cook food. If your microwave has power but is not heating properly, methodically testing parts like the door, fuse, magnetron, and control board will usually reveal the faulty component. Replacing inexpensive parts can often get a microwave working again and avoid the need for professional service calls or a complete appliance replacement.
However, caution is required when doing any microwave diagnosis or repairs yourself given the high voltages involved. If the issue remains unclear at any point, it is advisable to consult with a qualified appliance repair technician to properly evaluate and service the microwave.