Your experience just happens to exclude those disabling faults which are not reported by the idiot light, depending on the technology employed at the time. Otherwise you would not say it "tells you all you need to know."

--
Art Benstein near Baltimore

What engineers say and what they mean by it:
"Extensive effort is being applied on a fresh approach to the problem"
We just hired three new guys; we'll let them kick it around for a while.