Microwave Not Heating? Here’s 7 Simple Explanations & Fixes

Why You’re Microwave Isn’t Heating

A broken microwave, like any other appliance, needs to be diagnosed component by component.

For those unfamiliar (or need a refresher) on taking apart a microwave, here’s a great video which walks through step-by-step:

Onto the components.

I’ve listed these out in rough order of commonality.

Before we get started, anywhere I recommend to remove the case – make sure you disconnect the microwave completely from power first. Then discharge the high voltage capacitor before continuing. Again, if you don’t know how to do that, please don’t try to wing it!

Door Switches

The humble door switch, if failing, will disable an entire microwave. Since microwaves could harm us, it’s essential that no microwave operates when the door’s open.

This is more likely to occur on older microwaves, or those that often get slammed shut. 

We can quickly check this. When you close the door, does the light stay on? If so, the door switch isn’t sending the proper signal.

In high-end microwaves there may even be 3 or 4 door switch detectors. And it only takes one failure to shut-down the whole machine!

Diagnosis

First up, DON’T just try slamming the door harder! You’ll only cause more damage!

Instead, carefully but firmly close the door and re-check. If the door opens with a mechanical button, try pushing that down firmly as you close the door, then release when it’s closed.

Still no good? Then the switch is probably donezo. If you take the case off, we can check the terminals of the switch with a multimeter. Push down the switch to simulate the door being closed, and both terminals should have continuity (the read zero when tested). If you need a replacement, this will need to come from your manufacturer as each brand is different.

High-Voltage Diode

Your high-voltage diode plays a role in converting your A/C power supply into D/C power for the Magnetron. If this is malfunctioning (or has), you’ll likely have heard a buzzing or humming sound.

When inspecting it, it’s usually obvious if it’s burnt out!

Diodes can simply go bad over time, or from a bad magnetron. These are simple but delicate (and cheap) parts that are easy to replace. [Again: do not try if you’re not qualified to].

Diagnosis

These can be tested with a multimeter and a 9v battery. Multimeters channel electricity in one direction and should have a higher resistance on one end with a lower resistance on the other. If you find continuity with both sides, it’s no longer doing what it’s supposed to.

Replacements for most microwaves can be found for under $20 on Amazon.

Magnetron

Your magnetron basically IS your microwave.

This is the component that produces the microwave radiation which heats your food. It’s pretty important.

Magnetrons use a TON of power, and any malfunction will quickly blow a fuse. 

Diagnosis

These can be tested with a multimeter and should display a resistance of around 2–3 ohms. If that’s fine, then move one of the multimeter probes onto the metal casing. This shouldn’t display zero – if it does, something isn’t right.

If the fuse has blown, replacing the fuse isn’t a good solution. The root problem will still lie unaddressed.

You’ll likely need a new magnetron, and you’ll certainly need someone who can install it properly for you.

Thermal Fuse / Thermal Protector

As with any heating appliance, microwaves have overheating protection. Generally in the form of fuses.

Once blown, these need to be replaced entirely. While this is fairly simple, the fuses themselves are not nearly as common as the fuses in your home fuse box., so you’ll need to get the specific parts from a manufacturer.

To be sure about the condition of the fuse, check the mag and do a continuity test.

High Voltage Capacitor / Transformer

These work with the high voltage diode to power the magnetron. They’re some of the most complex parts to replace. And, unless your microwave is brand new and high-end, it’s probably worth considering a new microwave entirely.

Diagnosis

If it’s the transformer that’s gone, it will likely have arced (like in the movies) and produce a pretty offputting smell.

These would both definitely be noticeable if you were in the room when the fault occurred!

Control Board

The control board of a microwave is the same as a motherboard in your computer. In controls everything.

Which means, if it fails – you can’t control anything!

If it’s faulty, it’s doubtful the microwave will even turn on or have any functionality. Unfortunately, this is another major component which is very complex to replace. Unless your microwave is brand new, it may be worth considering a replacement.

How Does A Microwave Work?

Microwaves first came about in the 1940’s. When an American scientist noticed his radar equipment had melted the candy bar in his pocket.

These appliances have since revolutionalized kitchens around the world – but how do they work?

Actually, it’s fairly simple!

Every microwave contains a magnetron, which produces high-frequency radiowaves. Or ‘micro-waves’. This is similar to how radios and wifi work via waves in the air (which we can’t see).

The magnetron generates these waves and they’re directed by a waveguide into the metal box. They bounce around like crazy in there – especially when they come into contact with water. They have just the right frequency to be absorbed by water and making the water molecules ferociously vibrate. All this miniature level of movement gives out a lot of heat. Which is what cooks our food!

This is also why some materials, and entirely dry objects, are unaffected by a microwave. It’s also why microwaves are so dangerous to humans – since we’re over 70% water.

That’s great – but how does a Magnetron work? 

Great question, and it’s a little beyond my ability to explain well. Instead, here’s the best guide I’ve found to the physics behind magnetrons.

Conclusion

Microwaves have become an integral part of many of ours lives. And when they break, it can throw a real wrench into our plans for the day.

I hope this guide’s helped give you a few things to try if your microwave’s on the fritz. Again, do not try anything you’re not confident or qualified to do. If that’s the case, I’d stick to the original ‘silly’ tips and only let a technician take off the case and look under the hood.

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Thank you for taking the time to read this. I hope you have a fantastic day/evening!

-Craig