In most circumstances, a microwave will just stop together. When it has a problem heating, most causes will see it cease to run at all. However, if you hear it running normally, but pull out food that is still cold, this is a confusing problem.
While this is a weird symptom, it is still a symptom. You will have already narrowed down the problem to the heat-producing parts of the microwave.
You will want to discharge the capacitors for safety and get that multimeter ready for a lot of testing.
If your microwave is running but isn’t heating, you will want to test the following with a multimeter:
- High Voltage Diode
- Transformer
- Capacitors
- Relay
- Magnetron
While testing all these parts, you will also want to check them for damage or burn signs, particularly the magnetron. As the part that actually produces the heat, the magnetron should be a major suspect, but many parts contribute to its function as well. What can happen is that a magnetron can still test ok with a multimeter even when something is malfunctioning like the magnetron tube. You will likely still see scorch marks near the connectors if this is the case.
This repair, however, can be difficult. Many of the parts above can be dangerous to test if the capacitors are not properly discharged. Microwaves can hold a lethal charge even when unplugged in the capacitors. Furthermore, if your problem is the magnetron, it is expensive to replace. So expensive that it is usually more cost-efficient to replace the entire microwave.
As microwaves can be one of the more dangerous appliance repairs, it is strongly recommended that if the problem requires a lot of disassembly that you call a professional service.