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Faulty Catalytic Converter on 2005 Hyundai Elantra

About two months ago I bought a used 2005 Elantra from a reputable dealership. I had my mechanic friend check it out before I bought it and everything seemed fine. This week the check engine light came on. I took it to a shop (good reviews according to Yelp, but I’m new in town so my first experience with them) that charged me $145 to diagnose the issue and said I needed a new catalytic converter - his quote for the job was $1,300. As someone who knows nothing about cars, I’m wondering:

  1. Is it likely that the dealer knew about this issue prior to selling it to me?
  2. Is $145 a fair price to diagnose the issue?
  3. Is $1,300 a fair price for the job?
  4. How urgent is it that I fix the converter?
  5. What questions should I ask to get a better understanding of the problem and cheapest/best way to fix it?

Thanks folks!

9yrs is a lot of driving which could be a cause in cat failure. A california spec cat is $350. Any muffler shop can install it.

  1. If the check engine light wasn’t on, the dealer who sold it would have no way of knowing there was a problem.

  2. My mechanic charges $65/hr for labor. At that rate, your mechanic would have spent about 2 hours diagnosing the problem. Did it take him 2 hours? I doubt it. He probably checked the code, which took maybe 10 minutes. The code probably said “low catalytic converter efficiency.” And your mechanic assumed that meant you needed a new cat. Which it probably does not. More likely it needs a new oxygen sensor, which is much less expensive than a new cat. Ask your mechanic how he knows the cat is bad. Did he test all the oxygen sensors to verify they were good? Or did he just read the code and assume the cat was bad?

  3. The right question is “is it necessary?” Again, I doubt it.

  4. It’s not urgent. Do not allow anyone to sell you a new cat unless they can absolutely prove it’s bad, which I doubt. More likely it needs a new oxygen sensor(s).

  5. Ask your mechanic what codes he pulled, in “P0123” format and post them back here. Ask him how he knows it’s the cat that’s bad and not the oxygen sensors. If he said he tested the sensors, ask him how.

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Thanks for the advice! I’m heading to pick it up soon and will ask these questions and let you know what I find out. Car only has 79,000 miles on it so shouldn’t be due to wear from what I’ve read.

I agree, the cat is probably not bad. It’s most likely the O2 sensors. Get the code read at Autozone or Advance and post the codes back. Check Mechanics Files on this site. I have found it a great reference.

The car’s computer has O2 sensors before and after the cat. If the cat is working, the O2 sensor readings will behave in a certain way. The trailing sensor will track the leading sensor in an expected pattern. When the computer finds these sensor readings are not following the expected pattern it will flag the problem as the cat and turn on the CEL. But here’s the problem: if the two readings don’t track like they should, you don’t know if the problem is the cat being out of whack, or one or both of the O2 sensors being out of whack. There isn’t any simple way to test the cat, but O2 sensors can be tested to a degree. I doubt many shops do this tough.

$145 for the diagnostic seems a bit steep, but not entirely out of line. I expect if you were a long time customer the charge would have been lower. And $1300 for an OEM cat (including installation) is reasonable. If it is a non-OEM aftermarket cat, that seems a tad high. But not entirely out of line, depending on how difficult the access to the cat is.

Since you are looking at $1300 … hmmm … here’s some options …

  • ask the shop if they have a way to test the two existing O2 sensors.

  • ask the shop to replace the two O2 sensors with OEM new versions and see if that fixes the problem. I think that would probably cost $250-$350.

  • Opt for a non-OEM cat. Make sure it is legal in your state to do this though.

  • Just spend the $1300 and get this problem out of your way. If you do this, ask your mechanic to do a complete diagnostic trouble code read out and a fuel trim measurement to see if you have any misfires or an existing air/fuel mixture problem that remains undiagnosed. Those need to be corrected if you do, as they will eventually ruin the new cat.

Well it is the dreaded P0420 code. They tested both the cal converter and oxygen sensors as well, but it just seems to be the converter that’s faulty (the notes say poor mileage, P0420, efficiency below threshold bank 1, needs cal converter). Apparently it is the higher emissions standard type so the part would be something like $1,000 and the labor just $95. I am gonna shop around to see if anyone can do better on the pricing.

It sounds like they were honest, and did their job, but they’re probably using a cat obtained through the dealer. Dealer parts typically cost two to 2-1/2 times comparable aftermarket parts, simply because they have to go through the car manufacturer’s parts distribution system and then the dealer tacks on his significant profit.

Many folks lack confidence in aftermarket cats. There is some junk on the market, but most of the stuff out there is fine as long as it’s a “direct-fit OEM Replacement” part. The confidence failure is largely because some big-name chains put in their own one-size-fits-all crap, in some cases even welding it into the exhaust system.

You’re wise to shop around. Avoid chain shops, find a reputable independently owned and operated shop, and you’ll be fine.

The “dreaded” P0420 calls for more than merely checking the O2 sensors. And I don’t know how they would have literally tested the cat itself. Look at the potential causes of the code: Frankly the note about poor mileage makes me wonder about some of those things (coolant temp sensor, leaky fuel injectors etc.)

If no other cause for the code can be found and the cat does need to be replaced then you have to find out if something is amiss and caused it to fail. I have the original converter on a 1997 car with over 350K on it. They can just fail - but there is often a reason - e.g. running too rich or burning oil.

Sometimes used dealers put a 3mo warranty on. It’s often only drive train, but it’s worth a phone call to tell them about this and ask if they’d do a good-will check up on it.

It is unlikely the seller knew of this problem if the engine light took more than two days to come on after purchase. If you are paying real money for a converter be absolutely certain the replacement part is coming from the dealer.

Engines that burn oil like the one in your car will coat the converter with oil and ash so the chances of the new converter re-failing are 100 percent. This is why you want the best converter available so it lasts.

If you opt for the aftermarket chaeper converter it will refail in six to eight months whereas the dealer converter may last five years.

I have the same problem. Mine actually slows down from 60 to 40 then 20 and kicks in and speeds back up. I can drive it 20 miles or so and it will do it again. I was told it is the converter. I get free diagnostic testing in my area. And it said the oxygen sensors were bad. There are 3 converters for this car. May be 100.00 to 900. depending on which one. I have 224,000 on my Hyundai and I am just now having problems. Great little cars.