Let's get this clear - MICROWAVE OVENS ARE DANGERIOUS FOR 2 REASONS.
They operate with high voltages and they produce microwave radiation to effect the cooking.
Replacing INTERNAL electrical parts does not require radiation tests. The radiation is contained by the metal structure, the door seal and the holy shield in the glass. Damage or improper assembly to those parts would require the radiation tests. The door seal can go bad without any disassembly or be compromised by food stuck to it, so it is important to keep it clean.
Replacing INTERNAL electrical parts DOES REQUIRE that you know how to insure that the high voltage is removed.
As with ANY electrical device, NEVER work on it with power connected.
Even devices with batteries must have the battery removed b4 opening it up.
Sometimes, troubleshooting a device requires power when open. HOWEVER, never do that with a microwave because of the radiation danger.
A microwave that is powering on but simply not cooking is either the interlock switches or switch activators. the HV diode, the capacitor or the magnetron. Magnetrons are usually not economical to replace.
Any other strange operation might be the interlock switches, the switch activators, or the control board.
The control board can be replaced or if you are knowledgeable, a part, like the relays, might be replaced for a lot less money.
Just don't buy them from an appliance parts place, as they think they are made out of gold.