Old thread on the FTE (Ford Truck Enthusiast) forum that might get a few chuckles!

Ahoey said

# 1 rule of drag racing, change one thing it goes faster it goes slower at least you know which thing you did works.

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Correct. It works for some diagnosis, too. Sometimes you cannot test certain items. An ignition coil comes to mind. There's really no test for one. You could see it on a scope, I guess, but how many people have one in their back pocket?