Catalytic converter concerns - could it be because car sat for 6 months?
April 24, 2016 5:15 PM   Subscribe

I had a pre-purchase inspection performed on a used car I'd like to buy. The mechanic noticed a loud idle and some vibration at idle, and noticed a smell he said indicates a potential catalytic converter issue (I didn't notice any terrible sulfur smell, so I believe he picked up on a more subtle version of it). He mentioned that if the car had been sitting for a long time, that could be the cause, and would be fixed by regular driving. I'd like more information, but I wasn't able to find any resources related to "car has been sitting, catalytic converter suffers" or similar. What are the odds this is a real problem developing vs. could be fixed with regular driving?

According to maintenance records on CarFax, the car has been driven only ~100 miles over the past 6 months (this gap is logical based on the owner's reported history). Overall it's been very well maintained. Plenty of service records show timely maintenance, and the pre-purchase inspection turned up only this concern plus an unrelated and minor issue I wouldn't mind repairing myself.

There were no engine codes present when they checked it. The car was driven at least ~15 miles before the inspection. The mechanic thinks that if it is indeed a problem with the catalytic converter that it would be a problem that developed over the next year or so, not immediately.

Can anyone point to resources on this type of issue? Will regular driving fix this concern?
posted by reeddavid to Shopping (5 answers total)
What is the age of the car? What car is it? Mileage? It's impossible to give much of an answer without that.

It sounds feasible to me as a reason, and a catalytic converter is a wear/consumable item anyway so it wouldn't affect my assessment of the car, just factor into the price I paid. Price a new one and consider that against the offer you make for the car if you are otherwise happy with it.

Personally, I'd be more interested in the cause of the loud and vibratey idle. That is a symptom of something amiss to me. It could be exhaust not sealing until fully warm (but driven 15 miles should have fixed that) plus an injector/hydraulic lifter/electrical issue creating a loud and lumpy misfire or unrelated (engine mount) vibration and loud idle because of the car sitting (which a good thrashing and a good warm through would fix for sure).
posted by Brockles at 5:42 PM on April 24, 2016 [1 favorite]

Additional details I left out:

Car: 2006 Audi A4 2.0T AWD. 137K miles.
posted by reeddavid at 6:20 PM on April 24, 2016

Vibration at idle could be as simple as having 6 month old gas in the tank. Catalytic converter could cause a loud idle. Looking online, the catalytic converter is pretty cheap. An aftermarket one is $100 - $125, the Audi one will be much more. Also, make sure that the timing belt has been changed, it should have been done before 100,000 miles. A receipt is best but your mechanic should be able to tell if it's been replaced.
posted by Grumpy old geek at 6:50 PM on April 24, 2016

An AUDI with 137k miles on it? RUN AWAY!

Did you take it to Audi to have the plug it into the diagnostic computer? My sister had an Audi, about that vintage, and her 'Audi' mechanic screwed around with a problem, replacing around $1000 in parts and labor, before admitting that he didn't have a diagnostic computer and he was just trying things. When she went to Audi, she discovered that it had major problems, and ended up getting a new car because of it.

So spend the money for a real diagnostic with the Audi dealership.

Audis already have a well deserved reputation for not being very reliable after 100K miles. Their electrical problems are legendary.

If it's a powder blue convertible in Dallas, you DON'T want this car!
posted by Ruthless Bunny at 8:40 AM on April 25, 2016

So spend the money for a real diagnostic with the Audi dealership.

I'm not sure how many times we have to dispel this 'magical diagnostic computer' nonsense before you stop dropping it into every car related question, Ruthless Bunny, but please stop doing so. Your answer contains a lot of inaccuracies and one piece of anecdata.

There are no diagnostics available unless there is an existing problem. It is not an analytical device. If there is no check engine light on, there is likely zero information in the car of use at all that the computer can access.

To try and educate you on what plugging into a car can do: The check engine light indicates that an out of parameter event has been recorded. The computer (which doesn't at all need to be at a dealership, that is also wildly wrong in your answer - many, many mechanics have access to the codes in the car) that hooks up to it will receive a fault code (or maybe a couple of codes) that are produced by the event. Lots of different things can trigger the same fault code at times, and the source of the issue can be consequently many things and interpreting those codes (and maybe the set of codes is important) usually needs experience, external reference or additional testing. It does not tell you what is wrong instantly and it *certainly* doesn't give you any idea or assessment at all of the condition of the car.
posted by Brockles at 9:00 AM on April 25, 2016 [7 favorites]

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