WHEN GETTING A CAR DIAGNOSED?automotive mechanical?

i recently got my car diagnosed becuase my service engine light was on.. i went in because of a low idle and the car would die out.. the code read bad cam sensor... i know that the cam sensor can cause the problems im having ...the mechanic told me not to worry about that right now and change the spark plugs... i changed them myself and no difference whats so ever.. should i just replace the cam sensor or should i take his word for it again and see if theres another problem like m air flow iac or tps etc.... if the codes dont show up does that mean that there isnt a problem with other stuff??? is it possible for some older cars not to be able to be read on the diagnostics tester...???? its a 93 buick park avenue super ultra