Hi @packratfixer90 ,
What is the model number of the microwave oven?
If there is nothing wrong with it, leave it alone.
If there are definite problems with its’ operation that is a different matter.
What criteria are you going to use to determine which components are “failing”?
Just because a component looks “old” or ”faded” doesn’t mean there is anything wrong with it, e.g. a bulging or leaking capacitor is an obvious problem but then you could have a faulty capacitor which shows no external indications of a problem at all, and to test them you’d have to remove them from the circuit board.
You could cause more problems than what you started with (if any) when trying to conduct preventative maintenance on electronic components.
Mechanical components are a different story. e.g. if the turntable motor sounds as though it’s struggling or grinding etc then by all means replace the motor or if the microwave has mechanical timers or switches that are becoming unpredictable in their operation then again look to fix them, but as to the purely electronic components, (resistors, capacitors, diodes etc) wait until they actually fail or start to cause a problem before looking to replace them.