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I have a 2003 Volkswagen Jetta, which recently (at 110,000 miles) has developed an intermittent no-start condition. The engine will turn over -- seems the starter is working, but can't start the engine (similar to the situation when the ignition wires are bad). The ignition wires,plugs and ignition pack were all replaced several months ago.

I've made 3 attempts to fix it, but no lasting solution has been found. Per my mechanic a diagnostic scoping did reveal a code for an intermittent short and a faulty oxygen sensor. The oxygen sensor was replaced, but the no-start reappeared in a couple weeks. The fuel pump was changed. Again the problem reappeared in a couple weeks. Then a substandard positive battery connection was corrected, but the problem reappeared in a couple weeks. My mechanic said it could be some relays. What would you suggest trying next, as this is getting very expensive for me?

I was wondering if the fault could be in the computer, by natural wear or design.

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    Which model Jetta? 1.8T, 2.0, 12v VR6, or 24v VR6?
    – Lynn Crumbling
    Oct 17, 2016 at 18:47
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    You said this is intermittent. When this occurs, what do you have to do in order to get the car to start? Have you noticed any specific conditions when it happens? Is it typically when the engine is cold? Only after has been running for a while? When it is raining, sunny, etc. etc.
    – CharlieRB
    Oct 17, 2016 at 20:08
  • It is the 2.0L Jetta. Once or twice it started ok the following day, or after I had it towed to the mechanic's shop. Disconnecting the battery and reconnecting it worked once also. Disconnecting the battery clears the codes however; so I've avoided doing this so the mechanic would be able to scope the engine and better locate the cause of the problem. Type of weather does not appear to be a factor, as the car has started fine on several rainy days between attempted fixes. The last time the problem appeared corrected, but after four weeks reappeared.
    – Leif Ericson
    Oct 18, 2016 at 19:31

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The problem with intermittent problems is they are intermittent :) What that means is unless the technician can catch the problem when it's actually happening everything they check is going to check good. So in order to help you find the problem I am going to need you to get some starting fluid from your local store and keep it in the car. The next time it does it, spray a little bit of starting fluid in the intake while you have someone crank the car. If the car starts you have a fuel problem, if not we have eliminated the fuel system. Next see if you have spark while starting the car. Post your answer to this as a comment and we can go from there.

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Upon my mechanics later suggestion, he replaced the fuel pump relay. That appears to have resolved the issue as the car has started reliably for the last five months. I feel quite confident that that was the problem all along. The engine's analysis codes contributed greatly to the confusion, as they were somewhat misleading stating that the engine had 6 or so intermittent shorts to ground. Possibly this was triggered by the faulty relay. Anyways, thanks for the answer & suggestions provided in the above answer. I did not try starting fluid as the owner's manual advised against this. It took almost 11 months to get to the bottom of this problem, so I am relieved I did not give up or sell the car.

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