Microwave Not Heating? Causes, Checks, and Parts to Replace

When a microwave is not heating, it runs and the plate turns but the food stays cold. Usually, a failed magnetron, a bad high voltage diode, or a door switch is the cause. First, confirm the unit powers up and the door closes fully. Then, work through the high voltage parts below.

Safety first. Unplug the microwave before you open the cabinet. The high voltage capacitor can hold a lethal charge long after you unplug the unit, so it must be discharged before any internal work. Because of this shock hazard, leave the magnetron, high voltage diode, and capacitor to a qualified technician. Never run the microwave empty.

This repair involves the high voltage circuit, where a charged capacitor can cause a fatal shock. Have a qualified technician discharge the capacitor and test these parts.

Parts to inspect for a microwave that is not heating

1. High Voltage Diode

When there is no heat, a diode that has opened stops the voltage doubling the magnetron needs. The high voltage diode works with the capacitor to double the voltage that drives the magnetron. When it opens or shorts, the microwave runs but does not heat. A shorted diode often causes loud buzzing or arcing. Therefore, a qualified technician should discharge the capacitor and test the diode. A healthy diode blocks both ways on an ordinary meter, so it needs a tester with a nine volt battery in series, and then it conducts one way only. Replace the diode if it conducts both ways, stays open with that added voltage, or is blistered.

Danger: this is a high voltage part. Unplug the microwave and have a qualified technician discharge the capacitor first, because it can hold a lethal charge even when unplugged.

2. Door Switch

For no heat, a failed door switch can stop the magnetron from powering even when the unit lights up. The door switches tell the microwave the door is shut so it can power the magnetron. When one fails, the unit may stay dead or run without heating. So test each switch for continuity as the door closes and listen for the click. Replace any switch that does not make and break contact correctly.

Unplug the microwave before you reach this part, and have the high voltage capacitor discharged first, since it can hold a lethal charge even when unplugged.

3. Magnetron

When nothing heats at all, the magnetron itself is the prime suspect. The magnetron is the tube that produces the microwave energy that heats food. When it fails, the unit powers up and the plate turns, but nothing heats, and you may hear humming or arcing. Therefore, a qualified technician should discharge the capacitor and test the magnetron for continuity across the filament and for a short to the housing. Replace the magnetron if it reads open or grounded.

Danger: this is a high voltage part. Unplug the microwave and have a qualified technician discharge the capacitor first, because it can hold a lethal charge even when unplugged.

Get the parts

We supply OEM and replacement parts in bulk. Browse the Appliance Parts catalog, or request a bulk quote for volume orders.

Is the microwave old or failing often? So weigh a repair against a new unit, and browse complete microwaves and more when a replacement makes more sense.

Most of the time, a microwave that is not heating traces back to one worn part. So replace it, then run a short test with a cup of water and confirm the fix.

In short, a microwave not heating almost always comes down to the magnetron, the high voltage diode, or a door switch.