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.