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I replaced the main catalytic converter and manifold on my 2001 Hyundai Elantra, I believe it was last summer. A few weeks ago, I was on a long trip and the check engine light came on with the following code:

P0422 - Main Catalyst Efficiency Below Threshold (Bank 1)

I had noticed a decline in highway MPG from a typical 33-34 (loaded down with luggage, people, and A/C) to around 30 in the same conditions. Since the cat was fairly recently replaced and MPG was not what it should be, I assumed it was an O2 sensor malfunctioning and throwing off the fuel/air ratio. My understanding was that the O2 sensor in the manifold was the one that helped control the ratio, so I replaced it.

About 100 miles later, it threw the same code again, so I replaced the downstream O2 sensor. I assumed the issue was resolved since MPG appeared to return to normal. (I'm pretty sure the downstream sensor is just there to monitor, not feed information back to the engine, but it wasn't enough miles for me to refuel and check the MPG.)

Fast forward over 1000 miles and I've had no issues... until I did. It threw the same code again yesterday. A guy at Auto Zone told me to dump a Lucas fuel injector cleaner in the gas tank and reset the code after I've run through it. I haven't yet because I want to get some expert opinions, if I can.


A little research online has revealed to me that it's possible that the catalytic converter I bought was not up to snuff. I did buy it for a relatively low price on RockAuto.com. Apparently certain makes, such as Subaru and Toyota, are fairly picky about the quality of cat, but my research suggests that Hyundai is not quite so particular. Additionally, I've probably driven thousands of miles between replacing the cat and the light first coming on again.


Any insight you have into this issue is greatly appreciated.

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    Try cleaning the O2 sensor. It may be (but unlikely) that it is reading wrong. – Captain Kenpachi Jul 23 '15 at 15:17
  • Both of them are brand new (around 1000 miles), so I would hope that's not the problem. Would you still recommend I clean them? – Poisson Fish Jul 23 '15 at 15:25
  • Nah, it's a very rare problem. The best guess is that the new Cat isn't good quality or you are running too rich for some reason. If your car doesn't smoke or smell like rotten eggs, then it's probably the cat again. Do the injector cleaner thing anyway. It won't hurt. – Captain Kenpachi Jul 23 '15 at 15:34
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With the P0422 code, it can either be the O2 or the cat which is bad. Do as @JuannStrauss said at first and see if you can clean the O2 sensor. You don't do this by taking it out, but rather running some cleaner through the engine. My preferred method for this is a SeaFoam treatment.

If that doesn't fix your ills, you need to figure out if your cat is performing or not. One way to do this is to check its temperature. Not with a rectal thermometer, no. You want to use a digital laser thermometer. You'll need to check the temperature just before and just after the cat, where the tail pipe enters/exists it (right at the joint). If the cat is working well, the temperature after the cat should be about 200°F more than before the cat. If the temp is about the same or is distinctly lower, your cat is not working well and should be replaced. Should a new cat need to be replace after two years? In my thinking, absolutely not, but stranger things have happened. If you do need it replaced, it may still be under warranty, so check that out.

It could be that your after cat O2 sensor is bad once again. Some brands of O2 sensors just don't work well with some brands of vehicles. It may have become fouled out for some reason. This seems like the most likely issue here. I like the Denso brand of O2 sensors for several reasons: they are usually cheaper; they come with a small tube of copper anti-seize; they work well for their price point. O2 sensors should work for longer than a year also, but you never know.

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    Does the cat need to be warmed up, or checked immediately after starting, or anything special? – Josh Caswell Jul 23 '15 at 18:24
  • @JoshCaswell - Excellent question. The car needs to be fully warmed up. – Pᴀᴜʟsᴛᴇʀ2 Jul 23 '15 at 20:43
  • @Paulster2, here we are a few months later and it would appear as though the cleaner did the trick, thank you! I cleared the CEL after a tank of fuel injector cleaner and it hasn't come back since. If you happen to see this, one related question: at 170,000 miles, would you then recommend I replace the injectors? To the best of my knowledge, they have never been replaced. If the cleaner really did the trick, that tells me that the injectors might have some problems. Alternatively, I could just keep dumping cleaner in every few tanks, but I don't know if that is something to avoid. – Poisson Fish Oct 1 '15 at 18:31

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