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March 11, 2008 11:59 AM

Car Trouble Filter: Check engine light goes on. Mechanic says it the catalytic converter, cost = $1,500. Then the check engine light goes off, on its own. What should I do?

I have a '99 Subaru Legacy. About 100,000 miles, maybe more. The check engine light came on, and I brought it in to the mechanic the next day. He says it's the catalytic converter. Apparently there are two in the Subaru, and he also needs to put in a new oxygen (filter? detector? I forgot, but something like that) because he can't replace just the catalytic converter. I could just replace ONE of the two catalytic converters, but according to the mechanic, the second one could go and take the new one I replace with it, which would mean that I might end up having to replace both again. . .

Here's where it gets convoluted. I had JUST brought the car in for smog inspection (legally mandated where I live), just a few days before the check engine light came on. The car passed the inspection, no problem. So I find it weird that the catalytic converter could be malfunctioning, and still pass inspection just a few days ago.

And even more complicated. . . So the day after I get the quote from the mechanic, the "check engine" light goes off on its own. Huh? Now I'm more confused. Two days after that, the check engine light comes back on again.

What's going on? Any mechanics or someone with more car knowledge out there?

Thanks.

posted by jujube to grab bag (18 answers total)

Sounds like you should probably get another opinion from a different mechanic.
posted by gnutron at 12:00 PM on March 11, 2008

The CEL going on and off is an indicator of catalytic converter problems. My 2000 Outback had the exact same problem. I ignored it for a little while and then the DMV asked for a smog cert. as part of the renewal. It failed because the CEL came on again. Turns out that my oxygen sensor was busted. It was about $450 to fix (parts + labor + ruling out other issues + other stuff). This is in California.
posted by special-k at 12:08 PM on March 11, 2008

You need to find out the diagnostic code that's giving you the engine light. After that, you want to take that code(s) and run it into Google to decipher it. AutoZone will do this for you for free - if you are located in the US and in vicinity of an AutoZone.
posted by jerseygirl at 12:08 PM on March 11, 2008

Take it to Auto Zone. They will read the codes for you.

Which reminds me.... I need to do that.
posted by kuperman at 12:08 PM on March 11, 2008

I'd start by reading the OBD-II codes. If you have a national local auto parts store (Kragen, Autozone, Pep Boys, etc), they may be able to read your car's fail codes for free. The code reader tool plugs into a standard OBD-II port underneath your dash. That could give you an idea of what parts or systems were failing.
posted by jaimev at 12:10 PM on March 11, 2008

I'm definitely not a mechanic, but when my check engine light came on (VW Jetta) and I took it to the dealer, all that was wrong was that the gas cap wasn't on quite right (!?).
posted by violette at 12:20 PM on March 11, 2008

When I had a car with frequent problems, I bought a code reader. It was about $100, and saved me a boatload of time. There are plenty of things where the light will go on, the mechanic will tell you it's optional to fix and just turn the light back off. It's nice to see if it's that same thing, or something new without having to take the morning off work and rely on the mechanic to tell you the truth/change you $20 for plugging in a reader that cost $100.
posted by a robot made out of meat at 12:38 PM on March 11, 2008

On some (most) cars the light will trigger at 100,000 miles with absolutely nothing wrong. The trigger is a reminder to check your exhaust /emissions system. In some cases there is a switch under the dash to reset the light, on others it needs to be reset by attaching the code reader thingee and sending a reset command. I wouldn't do anything without a second or third opinion.
posted by Gungho at 12:39 PM on March 11, 2008

Nthing the O2 sensor. Also getting another mechanic after you get the engine codes from an auto parts place.
posted by kuujjuarapik at 12:41 PM on March 11, 2008

Be aware that the diagnostic code only tells you what part is failing, it does not tell you why. Replacing the part, if it is only a symptom of a different problem, may not be useful. You need an auto tech you trust to diagnose the problem.
posted by COD at 12:52 PM on March 11, 2008

Third or fifthing the ODBII reading. It'll tell you what the computer thinks the problem is.

For what it's worth, if the 1st and 2nd O2 sensor agree, the computer thinks your catalytic converter is done. It could be or it's possible one (or both) sensors are done.

Either way, from what I've seen of parts prices for your car, $1,500 is right on the money.
posted by jdfan at 12:56 PM on March 11, 2008

I just had my VW's engine read after the light came on, and got the P0420 code -- catalytic converter. Turns out there is a recall on my car to upgrade the software in the engine control unit, without which this code can occur erroneously. I realize you have a different model of car but a phone call to your manufacturer's customer support line might be able to tell you if there is anything like this outstanding for your car too. All I needed was my VIN number.
posted by galaksit at 1:11 PM on March 11, 2008

Buy your own code reader. I got one for about $50 at Autozone and it's really helpful; anytime you get a code you just google it or go to your favorite discussion board for your model car (they are TREMENDOUSLY helpful).

With that said, I drive a '99 Subaru Impreza. I had the P0420 "catalyst inefficiency" code (don't know if that is what you had) and it went away after I replaced my front o2 sensor (a 15-minute job for me in my garage and a sub-$100 part).

Take it to Autozone, ask them what the code is. Make sure you get the actual code in addition to what they tell you. Do some research on your own. Then take it to another mechanic and get their opinion. do NOT take it back to this guy unless someone corroborates his story because it sounds to me like he's full of crap.
posted by PFL at 1:35 PM on March 11, 2008

i just read a factoid somewhere that catalytic converters cost so much these days because of the precious metals inside the 'honeycomb' that is the guts of the device. I had a similar 'sticker shock' moment on my car recently and by shopping around on-line I was able to find a reasonably priced replacement (~$300) instead of the $1,000+ that i'd been originally quoted. my mechanic is an awesome guy who allows me to buy parts for him to install.

nth-ing the 'take it to another mechanic' - never hurts to get a second opinion!
posted by kuppajava at 2:20 PM on March 11, 2008

Have you checked with the Subaru dealer? The check engine light in my 03 Highlander came on early last year or so, and Auto Zone determined that it was an oxygen/emission issue. I ignored it, but a month later I received a note in the mail from the dealer about a recall on my model year Highlander due to defective oxygen sensors.
posted by Brocktoon at 2:49 PM on March 11, 2008

Hopefully you're taking the car to an independent Subaru mechanic, and not the dealer. I had similar problems with a '98 Forester -- the check engine light went on and off all the time -- and it was eventually diagnosed as a dirty oxygen sensor, which was cleaned for a few bucks and I was on my way.

Then the head gaskets blew, but that's a story for another day...
posted by M.C. Lo-Carb! at 2:54 PM on March 11, 2008

One more suggestion: after you follow the instructions above (get the code read at Autozone, have another mechanic look at it) then head on over to the "older Legacy forum" at legacygt.com and see what the locals think. It's fairly active so you should find someone who can tell you what that code really means. (Be sure to mention that it's intermittent.) There are also regional subforums that might help you find a trusted mechanic -- I'd link to the appropriate one directly, but there's no location listed in your profile.
posted by harkin banks at 6:31 PM on March 11, 2008

Thumbs up to the diy approach, esp if it's just changing the O2 sensor(s). The part is expensive and you might need a specific tool to remove it, but it's a great way to learn how your car works.

If it really is your cat conv. that has died you should figure out why it did so. It shouldn't have, unless something else is wrong. Overfilling the oil and using the wrong type of gas are possible reasons. Call around to some auto wreckers, you might find a used one for cheaper than a shop will charge you.
posted by drmarcj at 7:40 PM on March 11, 2008