That's what separates the mechanics from the true technicians. I call 'em parts changers.
I tell a rather humorous story about a Chevrolet product that ran very poorly when the coolant temperature was below 30 degrees F. It appeared to run excessively rich,so I investigated the fuel pressure first.I only had about a two minute timeframe before the coolant temp rose and the problem went away,so I had to work on the car outside in winter weather just to diagnose it. Typical throttle body fuel rail pressures for throttle body injection across all of the big three makers was a consistent 9-11 PSI. My test was showed a 32 PSI reading which sounded odd,because it wasn't normal and would cause an over-rich condition that was often the case of either a crimped return line or a stuck pressure regulator.
I hit my library of books and found two conflicting pressure stats,one book showed the usual 9-11 PSI and the other a 32-34 PSI rating. Still not convinced as to which book was correct,I hit the local Chevy garage and asked their head line tech what was correct. He told me that it was 9-11 PSI and if I didn't know that by now,I had no business opening the hood on a motor vehicle. I asked to see it in a GM factory manual. He got bent out of shape and pointed me in the direction of the service manager's office to take a look. I had never seen that 4 inch thick book,so I was looking in the index for the spec page.A few minutes later,Red stormed into the office and snatched the book out of my hands,flipped the pages to the spec chart and recited the 32-34 PSI to me. I said thanks and left,really pissed off.
As it turns out,upon further investigation of the ignition secondary voltage with my oscilloscope, a coil pack indicated that it was breaking down in the cold when the voltage requirements were up from firing a richer mixture when the engine block was below 30 F. I replaced both of the coil packs because they were in a miserable place on the rear of the engine and didn't want the comeback if the other one failed soon afterwards.
When I was satisfied that the car was repaired,I had the customer return to pick it up. I asked them what shop was the one that had changed all of the tuneup parts before they brought the car to me. The old tuneup parts hold all of the clues to what was the cause of the original problem,but since they were all replaced,the clues were gone. They told me it was Bodgewic Chevrolet and that the man who did the work was Red,the factory trained professional parts changer