On the way to work this morning the check engine light came on in my 2002 Hyundai Elantra Manual transmission. At lunch, I went out and had the codes pulled at Autozone. They said it was a P0420 error code. The code is "Catalyst Efficiency Low - Bank 1". In November 2011, I replaced the exhaust manifold, catalytic converter, front exhaust pipe with second catalytic converter & flex pipe, upstream O2 sensor, and downstream O2 sensor. I did all these repairs because the flex joint was leaking then when digging into Elantra issues, I read about the manifolds cracking. My exhaust manifold did have a crack in it. I replaced the O2 sensors because I was replacing everything else and the car had more than 100k miles on it.
What are your thoughts for the possible causes of the P0420 error code? What should I check?
Yours is a 2002 so it probably falls within the cracked manifold TSB. It should be in here somewhere. Chances are the dealer already checked it at some point, which is fine because they warranty it from that point. I don't remember the details (I recently had mine replaced) but I think you'll be covered.
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I got the car home and started tearing into it. No visible issues (exhaust leaks) on the underside of the car. I pulled the front exhaust manifold heat shield and removed the upstream O2 sensor. The sensor looked decent but not great. It was grey. After I had the O2 sensor out, I could see that there was a bolt missing on the exhaust manifold to cat flange. I'm hoping this was my issue. Exhaust leak causing the O2 sensor to think it is lean and setting the code. I'm in process of replacing the broken stud.elantra gls missing exhaust stud.JPG
I replaced all four studs with flanged hex head class 10.9 bolts. I left the front heat shield off for now so I can perform a quick inspection by raising the hood. The error code reset since I disconnected the negative battery cable. On the way to work this morning, no error code popped up. I hope this cured the issue.
just fyi, DTCs are set when the issue happens 2 consecutive times. it may take a coupl drive cycles for the ECM to test the sensors.
and yes, the missing bolt would likely cause the o2 sensor to read lean.
The same code is back. After 130 miles of driving, the P0420 code came back. I think my next move is to replace the downstream O2 sensor even though it was replaced in November 2011. I could take the car to a shop to have the O2 sensor graphed. If the shop graph shows that O2 sensor is NG, then I could get a warranty replacement from Autozone but I doubt I would get reimbursed for the shop fees to graph the O2 sensor.
I had a similar problem and replaced the downstream o2 sensor twice (about $100 a pop!). I ended up replacing my entire exhaust front to back, but the original fix was a magnaflow exact fit replacement exhaust manifold/cat. It's possible you got a ripoff cat and it didn't have enough plutonium in it. I seriously doubt your o2 sensor has failed, don't waste the money. After I replaced the manifold I still got o2 codes and it turned out to be the flex pipe. Good luck!