You asked him to do something that is almost impossible to do, especially with intermittent problems. When the engine is running, there is no defect to be found so no matter where or how hard he checks, the computer is doing what it's supposed to do, ... Running the engine. Engine Computers really don't cause as much trouble as most people think. GM computers from the late '80s were the exception. Mechanics changed so many of them that that's the first thing many of them jump on today.
When you have a no-start condition, it's rarely caused by the computer but when it is, you know it as soon as you plug in a new one because the engine fires up. With intermittent problems that are hard to duplicate when you want them to act up, all we can do is try the most likely causes to eliminate them as suspects, THEN assume it's the computer after there's nothing else left. We have the same trouble with integrated circuits in tvs and vcrs, There is no way to test them. We have to check power supply voltages, grounds, input signals, and be sure the outputs aren't shorted. That takes a long time, but then we can make an educated guess that IC is bad if everything around it is good but it doesn't do what it is supposed to do.
The other problem with intermittent problems is you can never know it's fixed if it doesn't act up after you replaced or did something. If it acts up again, you know it's not fixed. But if it doesn't act up, is it fixed or it just hasn't acted up yet?
Don't be too hard on your mechanic. The only way he could have done what you requested would be to send your old computer to a rebuilder with very expensive test equipment dedicated to your specific computer, or he would have to have a known good used one on hand to plug in and try. As you can see, that still won't tell him definitively if the old one was bad.
Tuesday, November 6th, 2012 AT 9:47 AM