A powerful troubleshooting technique when working on microwave oven that won't heat is to split the problem in half to determine if the problem is coming form the low voltage side or the high voltage side. A test cord can be a very useful troubleshooting tool because it allows you to so just that. Sublime Master Chat_In_RI enlightens us on this technique:

Why can't you make a test cord to power the M/W?

No model number here, but a test cord to the primary side of x-fmr to by-pass door switches, relays, etc... and if your diode and cap check out OK, narrows it down to the mag if no heat.

Source: GE Double Oven Microwave not heating