Now you see it, now you don't. The tooth-ache that disappears as you get to the dentist office. The funny noise in your car that won't happen when you take it to the garage. It's called an intermittent problem, and it is extremely common in modern cars. Sounds almost like an oxymoron, I know, saying a problem that isn't there is a common problem, but it is the unsteadiness of the problem that is the common thing, not the problem itself.
In the old days, before computers were installed in cars, most problems were dependable. That is, they stayed there while you got help, and the mechanic could observe it himself, the same way you did. The car wouldn't start, or made a noise or a vibration, and kept doing it until it could be fixed. Then along came computers. You know how your home computer sometimes freezes-up, and you have to reboot it to get going again? The same thing happens with the computers that are controlling our cars. There could be a problem with the hardware, or the software, or an outside influence, like a voltage spike could be the culprit. Rebooting cannot fix anything, but if the problem was an outside influence, it can make the computer ignore the interruption and continue like nothing happened. So when your engine computer locked up, your car died on you. You turned the key off and tried to start it again, and it started fine, because turning it off and on caused it to reboot the computer.
Now you take it to your repairman and ask him to fix it because it died on you. Is it possible to diagnose an engine for stalling while the engine is running fine? Maybe, and maybe not. Those pesky computers do have some memories of what happened to them, but it is limited. It may remember that it got a voltage spike which caused it to fail, but sometimes its' memory is blank. In this case, the problem is a mystery to be solved, with precious few clues to go on. If the problem doesn't occur while the technician is diagnosing, and the computer systems do not remember what happened, the tech simply cannot be sure of the cause. It seems the natural human tendency of car owners in this situation tends toward anger, sometimes at the car, sometimes at the tech. Anger never helps. Actually it makes things worse. Both the tech and the owner are under stress. The tech needs to get paid for the time and effort he has devoted to the car, even though he is unable to isolate the problem, and the owner doesn't think he should have to pay, because nothing was fixed. Add anger to the mix and the pot can boil over. What's needed, of course, is understanding. The tech usually understands the owner's point of view, because he sees it nearly every day, and the owner needs to understand that the problem may never be diagnosed, unless it finally stays in evidence long enough to be diagnosed, and if it doesn't, it's not the tech's fault. The tech, as the focus of the owner's anger, may not wish to work for that owner any more, and the owner may go away mad, in which case the whole thing was a waste of time, and it was really only a problem with a machine to start with.
There are two ways to handle intermittent problems. Either wait for it to get worse, to aid the diagnosis, or make guesses about possible causes, known as "throwing money at it." An angry response is not a viable third method. "Patience is a virtue, often in demand, found seldom in a woman, and never in a man."
Wait, I may have that backwards. Let's see now..