Engine Trouble Code False Positives Causing Unneeded Work?

I brought my Passat in to my local VW dealer for a 120,000 mile service. The check engine light was on and had been on for about 4-5 days prior to the service. They did the 120,000 mile service. They recommended doing a complete cooling system flush which I agreed to. They also told me that the car’s computer trouble codes indicated that one of my catalytic converter’s needed to be replaced at a cost of $1100. Being on a budget I said no to the catalytic converter. After about 24 hours the check engine light went out and has stayed out ever since. This is about 3 weeks now.

My question is how precise is the info given by the car’s computer? How much is subject to interpretation by the technician? Does my converter really need to be replaced or was this just a false positive. And if this was a false positive how often do people end up paying for repairs they really don’t need. I ask all this because this is the second time my car’s computer has said that some expensive electronic module needed to be replaced. The first time this occurred I had some routine maintenance done including replaced some leaking vacuum hoses and that seems to have fixed the problem negating the expensive part’s replacement. That first time it was at an independent repair shop and the manager told me to wait on replacing the expensive part and see if the other repairs solved the problem which it did. I got no such advise from my VW dealer.



“I got no such advise from my VW dealer.”

…and thus, you see the problem with many (perhaps most) dealerships in general, and VW dealerships in particular.

Did you ever wonder why there are more independent VW specialists in most areas, as compared to independent specialists for other foreign makes? That is undoubtedly because VW dealerships consistently receive the worst ratings for customer service of all new car dealerships in surveys such as the ones done by Consumer Reports magazine.

As to what codes may indicate, in general they point to a probable problem area, but they do not necessarily indicate that a particular part needs to be replaced. However, if a dealership is essentially a “parts changing” establishment–which essentially all of them are–then they will replace a particular part, even if other, less expensive repairs might have resolved the problem.

In the case of your catalytic converter, it is possible that you have a flukey O2 sensor, and that replacing this relatively inexpensive part would resolve issues that the dealership believes could only be resolved by replacing the very expensive cat converter. Wiring issues might also be a less-expensive fix for this problem.


I suspected that something that got replaced as part of the 120,000 mile service or even the coolant flushing fixed what the Check Engine Light (CEL) was complaining about. Thanks for explaining that the code indicates a problem area and not a solution. So there is much that is subject to interpretation by the technician. At this point I expect that the CEL will stay off.

There is no code that indicates a catalytic converter needs to be replaced. There are codes that indicate the catalytic converter is not working optimally, but that can often be corrected by things that cost much less than a cat.

The dealer is out to maximize profit, which is why they always recommend the expensive solution to a problem.

The cooling system flush is routine maintenance and was properly recommended. It had nothing to do with the trouble code.

The value of the trouble code is dependant on the technician. The code itself will only be triggered by the ECU comparing the signals two oxygen sensor readings. If the two readings, before and efter the cat converter, show the converter to be operating below efficiency, the tech should then look at eth signal traces from the actual sensors. Those traces can help him determine if it’s the actual converter that’s bad or just the sensor. If the inefficiency of the cat converter is marginal, the light can be triggered under a specific condition and then reset. That does not mean the problem has gone away, only that it was marginal to begin with. It isn’t really a “false positive”.

It sounds like the independent repair shop served you well. If it were me I’d patronoze tham rather than the dealer. Even honest dealers are restricted to using their franchiser’s supply system for parts and these can often be considerably more expensive than aftermarket parts.

Thanks for all the info,

Normally I do patronize the independent repair shop and have been for the past two years after the local VW dealership started recommending additional maintenance work that wasn’t in the manufacturer’s list of recommended maintenance. However, this time my independent shop was way more expensive than the dealership for the 120,000 mile maintenance and I could not afford the additional hundreds of dollars.

Thanks for the detailed explanation about the before and after O2 sensors. For whatever it is worth I filled out VW’s satisfaction survey and I dinged them on the catalytic converter. Now I’m getting emails from the service manager. Now I have some very specific ammo to go back to him and explain that no one explained what was going on and that it might be the sensors and not the converter. It will be interesting to see what he says. I’ll post it if its worth a chuckle.


Let me sound out this logic, the light comming on is not good enough to conclusively say a part is bad (I agree) and the light staying out is good enough to conclude a part is good? this is the part I have trouble with. Perhaps some additional testing was done (like a simple comparison of inlet and outlet temps on the cat.)and this is what made the shop comfortable in telling you your cat. was bad. There are tests that can move conclusions about potential problems identified by the cars OBDII system more firmly into the “good” or “bad” catagory, but these tests cost. At times the cost is minimal but the mechanic still wants to be paid to perform them. I do think that every cat. and 02 sensor that is a part of a system that has caused a code to be generated should be subjected to additional confirmation tests. Lastly I propose that perhaps the Dealer was working off the experience gained from seeing so many of the same type car/failure. The customer is always allowed to ask for additional confirmation testing to be done, perhaps the shop will do this confirmation testing free of charge (to the customer) but still pay the mechanic something,hopefully.

The problem is the dealer’s service rep didn’t explain anything about how and why they concluded I needed at $1100 catalytic converter replacement even when I questioned it. I guess that’s a training issue for their service reps. If they had explained what various people here have explained to me I would have considered replacing it when my budget allows me to. With the CEL dark I’m proceeding assuming the converter is worked well enough. If the reps cannot explain to me how they decided it needed to be replaced why should I believe them. Especially now that the light has gone out and not come back on. I think you give them more credit than they deserve. I had similar problems when I was dealing with flooding inside my car from backed up sun roof drain vents. After some research online I knew more than they did.