Hopefully, you didn't replace these parts as part of a troubleshooting technique.
 

My only question is: When is parts replacement as a troubleshooting technique finally stopped, and why and how?
It seems almost everyone climbs on, and goes down, this Slippery Slope.

I guess I am not a gambler. Maybe it has to do with gambling small amounts at first and then betting the farm on the control board.

What holds me back is that I know it could be something as simple as a corroded connection that could be fixed with fine sandpaper, parts cost = zero.
And using a $20 multimeter considerably narrows the choice of what is wrong.

Nope, trouble shooting is the step before you start replacing parts. Are you seeing more people changing out parts before they figure it out?

Seems they all do, even if expert advice is available on forums and in books and instruments are available in stores.

I can see taking one guess, but to keep buying parts that don't fix the problem seems not too rational.

Doctors bury their mistakes and I guess the equivalent for this is to buy a new furnace.

If you don't know where the problem is all you can do is buy tools and gimmicks and start without thinking. Buying parts is just a result offered by people that sell parts and may be willing to sell parts at higher prices while guessers try to learn something from the process of buying.

Dick

People are not making full use of the credible information available to them.
http://en.wikipedia.org/wiki/Troubleshooting

That's an interesting read.