Hello. I have a less then 3 year old above the range microwave that
all of a sudden wont heat up. No flashes or pops, it just stopped
heating up. Everything else works, turns on, all the buttons, lights
and fan. I noticed that although my warranty was only one year that
the magnetron tube is under warranty for 8 years. Would the magnetron
tube cause it not to heat up?

Yes, a bad magnetron is one possible reason why your oven isn't heating.
However, there are numerous other reasons that could cause this as well. It
could also be a bad high voltage capacitor, bad diode, faulty door
switches, faulty control panel, burned wire, etc. Magnetrons "usually" hum
when they go bad, but not always. Microwaves store dangerous voltages even
when unplugged, so they're not really a fix-it-yourself item, even if you
can find a source for the parts. It's probably cheaper and easier to
recycle the old oven and buy a new one.

Anthony