Our traditional range or wall oven may dominate the kitchen in size while providing us with the most versatility in baking and broiling, and a toaster oven certainly helps out with reheating pizza slices or warming up bread and rolls, but neither of these appliances can save time as a microwave oven can. When you need to throw a meal together quickly, nothing cooks faster than a microwave… until you discover it’s no longer heating when you press the start button.
What’s the cause of a microwave not heating? We’ve got seven probable causes for you four paragraphs down if you want to skip ahead. But first, let’s examine how a properly functioning microwave heats up our food…
Microwave oven operation 101
Although microwave ovens differ in size, appearance, and output wattage, they all operate on similar principles. Once a designated power level option and cooking time is selected, pressing the start button will cause the appliance’s control board to send 120 volts of alternating current (AC) through a line fuse, a cavity thermostat, and multiple door switches, before the current reaches a high voltage transformer. A high voltage capacitor works with a diode to convert the alternating current power output of the transformer to direct current (DC), doubling the voltage to nearly 5,000 volts, which provides the voltage needed to power the appliance’s magnetron. The magnetron emits radio waves that travel through a wave guide before entering the oven cavity.
Once inside the oven cavity, the radio waves cause water molecules within the food to vibrate intensely, generating heat that cooks the food from the outside in. Most microwaves use a motorized turntable to rotate the food items through the radio waves in order to cook those items evenly. Keep in mind that the heated water molecules will steam the food items during the cooking process which is why bread tends to get soggy when heated in a microwave (let the toaster oven take care of reheating bread products).
When operating, the high voltage components can generate considerable heat, so the control board also sends voltage to a fan motor to cool the appliance. One or more thermostats will prevent overheating by shutting off the voltage to the high voltage system if the microwave’s cabinet or oven cavity area get too hot.
Why won’t the microwave heat up?
If that pasta primavera is coming out of the microwave as cold as when you put it in, here are the seven most likely reasons why:
- Burned-out diode – Since the diode is the component that doubles the voltage to power the magnetron to heat the food, a burned-out diode will result in the magnetron not receiving enough voltage to operate. When a diode fails, it will often appear visibly damaged. You can also test the diode with a multimeter and a 9-volt battery to determine if the diode has electrical continuity – a continuous electrical path present in the part. Be aware that the diode should only have continuity in one direction. If the diode either tests “negative” or “positive” for continuity in both directions (when the multimeter probes are reversed), then the diode has failed and will need to be replaced.
- Defective door switch – Most microwaves have three or four door switches that allow the microwave to start or heat once the door is closed. A defective door switch can prevent the microwave from heating even when the door is fully secured. As with the diode, you can use a multimeter to test each door switch for electrical continuity to determine if one or more have failed.
- Malfunctioning magnetron – If the magnetron itself is malfunctioning, the microwave will not heat. Again, you can use a multimeter to test the magnetron for electrical continuity to help determine if the part is malfunctioning. The reading should be less than one Ohm and the magnetron should not have shorted to ground for the component to be working properly. Since the magnetron is not repairable, a burned-out magnetron will need to be replaced with a new one.
- Faulty high voltage capacitor – Since the capacitor works with the diode to convert and double the current sent to the magnetron, a faulty capacitor will prevent the entire high voltage circuit from working. When a capacitor fails, it will often bulge or leak. Even if you don’t see any visible signs of failure, you can test the capacitor using an analog-style Ohm meter or a capacitor tester to determine if the component is capable of storing and releasing its charge in addition to determining whether it is operating with an acceptable capacitance rating. Just be sure the capacitor is discharged before handling it (see below).
- Failed high voltage transformer – The high voltage transformer may be failing to power the magnetron. When a transformer fails, it will often arc and produce a burning smell.
- Blown thermal fuse, cavity fuse, or thermoprotector – All three of these components are designed to cut off the power to the microwave if the appliance overheats. Once more, you can use a multimeter to test each of the components for electrical continuity to determine if any of them have blown and require replacement.
- Defective main control board – The main control board regulates the power supply to all of the microwave components, so a defective board might be causing the appliance not to heat. Since this is not a common problem, you should inspect and test the other potentially faulty components first to confirm they are working properly before considering a control board replacement.
Put safety first when repairing a microwave
Because of the high voltage and high current used by microwave ovens, be aware that repairing this appliance poses a substantial risk for injury or death if precautions are not taken. You should always unplug the microwave before you attempt any disassembly. Since high voltage capacitors used in microwaves may retain a charge even after the microwave has been unplugged, we recommend that only experienced professionals access and replace internal components.
If you do need to access internal components, you should make sure the capacitor is fully discharged immediately after removing the microwave’s shroud or cover. While most high voltage capacitors used in recent microwave models have a bleeder resistor in them that should automatically discharge the capacitor after the voltage has been discontinued, it’s always a good idea to confirm this by attempting to discharge the capacitor manually. This can be done by placing a screwdriver blade or needle-nose pliers across each set of capacitor terminals. The handle of the screwdriver or pliers should be insulated, and you should avoid touching the metal portion of the tool when the tool is in contact with the terminals.
Find genuine OEM microwave oven parts with Repair Clinic
Did your troubleshooting reveal that the diode or magnetron has burned out? Or, perhaps, a door switch is defective, or the capacitor is failing? Then the next step is to find the exact replacement part that works with the microwave model you’re repairing. This is where Repair Clinic.com can help out as your repair partner. Enter the full model number of the microwave in the Repair Clinic website search bar to see a comprehensive list of compatible parts. You can then use the “Part Category” filter (examples: “Diode, Magnetron & Resistor” and “Switch”) along with the “Part Title” filter (examples: “Diode” and “Door Switch”) to identify the specific part you need. Repair Clinic stocks original manufacturer replacement parts that match the most popular microwave models such as those built by GE, Samsung, Whirlpool, LG, Kenmore, Frigidaire, and Panasonic, in addition to offering a wealth of free microwave repair help content in the form of part testing and step-by-step procedural videos, articles, diagrams, and schematics.