Because there is a difference between simply reading the codes, and everything that might be available from the computer to help diagnose rather than simply throw parts at a problem.

I go to AutoZone and they give me about 2 feet of printout of ALL the possible sources. I want to know which of 12 possible components is giving out of spec readings.

__________________
1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags