Q – My 2006 Carrera has had a recurrent check engine light issue and the code that always set was for the camshaft position sensor. I would have the computer reset and a few days or even a week later, the light would come back on.   My mechanic suggested either testing or a pre-emptive part replacement.   I decided to just have the part replaced.   As I was on my way to the shop, it began to stalling at idle but it always started back up and drove fine. The puzzling part is that when I dropped the car off for the cam sensor, the car would not start and now I am told that it has a failed crankshaft position sensor. They also told me that it could have a camshaft position sensor problem but until the crank sensor is replaced, they can’t say with any degree of certainty. Although the warning for the cam sensor has been recurring for a few months, the car never ran poorly or stalled. What do you think?   Jerry

A – This is a good example of why you can’t trust computers. There, I’ve said it. I admit I am a dinosaur when it comes to computers. I know who to ask and here is the word: The computer expects to see the position of the camshaft relative to the position of the crankshaft and when it does not, it sets a code. It could also set a code if the sensor failed altogether but still the car would run. It will not run with a failed crank position sensor. Of all the sensors on the car, that is the one that will strand you alongside the road. Computerized cars have the remarkable ability of adding in a missing signal as a default if a sensor fails which then allows the car to run, albeit poorly. This electronic house of cards comes tumbling down when the computer cannot detect the crankshaft rotating or it’s position relative to top dead center (a critical reference point).. The camshaft position, while very important, is not a deal breaker but should not be ignored. The output of both sensors can be verified with an oscilloscope which is a fairly standard shop tool these days.

The odd thing is that there was no code for the crankshaft sensor. Had you replaced the cam sensor, I am sure the car would have stalled, to the great embarrassment to the unwitting tech who did what the computer indicated he should do.  Replacing the failed crank position sensor will probably correct the computers impression that there was a problem with the cam sensor provided that the cam is working properly and provided that that sensor passes the oscilloscope test.   Those dog gone computers!

So it’s a blessing! The car failed at the shop, on the tech, not you!   You did not get stranded, the car did not need to be towed, you didn’t have to pay for a part that would not fix the car and, I trust, you have had many trouble free miles with no check engine light.   MC