Microwaves contain high-voltage parts that can be deadly, even when unplugged, unless handled correctly. Unless you are a qualified expert, don't attempt to get into its inner workings.
First, use this water method to double-check if there is actually an issue with your microwave not heating:
1. Fill a glass measuring cup with water to the 1-cup mark.
2. Set the microwave to heat on full power for 1 minute.
3. Repeat the process with fresh water a handful of times.
4. If the water is not consistently warm, your problem is confirmed.
If your microwave is failing to heat correctly, you may be on a previously programmed setting. Press the STOP/CANCEL button and start the heating process again, but use a higher power setting. This may correct the problem.
If the microwave senses an open door, it will not run. Check to see if a defective or loose door-safety interlock switch is to blame. If tightening it doesn't work, a replacement may be necessary.
Did you know? Neglecting to hit the STOP/CANCEL button on the microwave before you open the door to remove your food damages the safety switch over time.
Microwaves installed above the range may be affected by steam from cooking. Condensation collecting on the circuitry can interrupt heating. Allow the circuitry to dry out by avoiding stovetop cooking for a few days. If you're still having the same problem, it could be a loose connection in the circuitry or a faulty door safety switch.
There may be an issue with the circuit board and its connections, and that work is best left to a qualified professional.
A buzzing or humming microwave that emits a faint burning smell could signal a damaged magnetron, high-voltage capacitor or diode. Replacement magnetrons are expensive, so if a service professional determines that's the problem, you may be better off buying a new microwave. Capacitor and diode parts cost much less, but remember, you'll be paying for the expertise to have them installed correctly and safely. It may still make more sense to buy a new microwave, especially if it's a simple countertop model.
For more troubleshooting steps and simple solutions, read Microwave not heating
. I hope this helps!