Microwave Runs but Does Not Heat? Here’s Why

If your microwave turns on, and the carousel spins, but it’s not heating the contents much or at all, there are steps you can take to attempt to fix your appliance before outright replacing it. 

While there are a few potential causes, one of the most common is that the microwave’s power levels are set to the lowest preset. That’s one simple thing that will prevent your microwave from heating properly. Otherwise, the other causes will require you to open up your appliance’s internals. 

A Note on Safety

Microwaves are equipped with large, high-voltage capacitors that store electricity. These devices can retain a charge even when the appliance is powered down and unplugged. So, before conducting any repairs, it’s important to discharge this capacitor. The best way to do this is with a screwdriver or pliers with an insulated handle. If you don’t feel comfortable doing this yourself, the repair might be best left in the hands of a professional. 

Similarly, a faulty magnetron is a potential reason your microwave isn’t heating. Magnetrons are the components that generate microwaves. Among the many materials used in their creation, magnetrons contain beryllium oxide, which is dangerous if inhaled. Though this is unlikely, it’s still a consideration. 

In addition, a broken magnetron cannot be repaired and must be replaced. This process is expensive and requires a professional, so a new microwave is typically the better alternative. 

However, the good news is that there are simpler issues that might cause your microwave to not heat properly. Let’s start with the most basic.

Door Switch Malfunction

Since the waves of the microwave itself heat up just about anything they touch—for better or worse—the appliance won’t run if the door isn’t closed. So, a broken door switch will prevent the appliance from turning on, even if the touchpad still seems to work. 

Before taking apart your microwave and replacing the switch, it’s good to make sure that the latch itself (the part of your door that actually makes contact with the switch) isn’t the reason the door isn’t closing properly. If, for some reason, the latch’s prongs aren’t making contact with the switch—due to damage or debris—the switch won’t be able to tell whether the door is closed. 

You may be able to realign or clean the prongs so that the latch contacts the switch, and the microwave will begin turning on and heating properly. However, if the latch seems to be working fine, you’ll need to test and potentially replace the switch itself. 

Before beginning, ensure that the microwave is unplugged and that the capacitor is discharged to avoid electric shocks. In most models, the switch is near the latch, and some models have multiple switches. To determine whether this is the reason your microwave isn’t heating properly, test each switch with a multimeter or ohmmeter to see whether they have continuity. 

After setting the meter to resistance, you’ll need to test each of the switch’s terminals. Without pushing on the actuator, the multimeter should give a reading of infinity, showing that there is no continuity. Then, when you press the actuator—it might or might not “click,” depending on the type of switch that it is—it should read zero or close to zero, indicating that it is working. If it’s still showing no signs of continuity, it will need to be replaced.

To replace the switch:

Tip: Take photos for each step to help when installing the new switch. 

1. Remove the wiring attached to the switch. 
2. Unscrew the switch assembly located on the side, sliding it out of place as necessary.  
3. Detach the connectors. 
4. Unscrew or unclip the faulty switch and remove it. 
5. Screw or clip in a replacement switch. 
6. Reassemble the microwave, plugging in all the necessary wiring and screwing in the brackets and face that was removed. 

Faulty High-Voltage Diode 

If your microwave’s inability to properly heat is accompanied by a burning electrical smell, there’s a good chance the high-voltage diode has gone out. This component provides power to the magnetron, so if it’s not working, the microwave will run but not heat. 

To test the diode, start by unplugging the microwave and discharging the high-voltage capacitor to avoid potentially deadly electric shocks. Then, using extreme caution, disconnect the leads from the capacitor. If the diode shows any signs of damage (or the aforementioned smell), it’s most likely defective. To know for sure, test it with a multimeter; a working diode should read anywhere from 50,000 to 200,000 ohms. 

Replacing the diode is relatively simple since most have a press fit or ring lugs. Just be sure to proceed cautiously to avoid damaging the connectors, the capacitor, or the new diode. To find a replacement, match the diode used in your microwave’s model with a new one. 

Outside of that, ensure that you’re getting the polarity correct. Otherwise, the diode won’t work. Just remove the old one, and slot the new one into its place. While you might need to solder or weld the leads to the diode, you might be able to crimp them instead. Just don’t use too much pressure while doing so. 

A Bad Magnetron 

Lastly, if everything else about your microwave seems fine—switch, magnetron, settings—then there’s a good chance that the magnetron has gone out. As we mentioned earlier, the magnetron is an expensive component, so if you’ve isolated it as the cause of your heating issues, your best option might be to simply get a new microwave. 

However, like any component, they can still be replaced, especially if you have access to spare microwave parts. Just make sure to find a compatible magnetron for your microwave’s model. 

To test your magnetron, start by unplugging the microwave and discharging the high-voltage capacitor. Like the previous components, a multimeter will allow you to test its continuity. This will require two separate tests.

First, test each of the magnetron’s terminals with the multimeter set to ohms at the lowest resistance. Then, reverse the meter’s probes and test again. All tests should be close to zero ohms. 

Second, start by raising the resistance to the highest settings. Touch one probe to the magnetron’s metal housing and the other to one of the thermals. Do not touch the probes together, as this would provide incorrect readings. This test should produce a reading of infinity, indicating an open circuit.

If your magnetron failed one or both of those tests, it will need to be replaced. While we don’t recommend sourcing a new replacement, if you have a spare part handy, you can replace it.

Once again, a great tip is to take a picture of the original magnetron to ensure that you connect the new one correctly. After disconnecting the wires, unscrew and remove the old magnetron, then slot in the new one, using the photo to reattach the wire correctly.