My Sonata has 117,000 miles on it.
2 yrs ago we installed a hydrogen fuel efficiency unit. However, it kept on blowing the safety fuse, so we disconnected it 2 months later, never used it since.
For this unit to work we had to bypass the O2 sensor so as not to get a false read. Since then I suspect the O2 sensors are not happy.
The car just got a tune-up, new head gasket, spark plugs: is overall well-maintained.
I live in Baja, Mexico now, and the roads are rough and dusty here. Recently the engine light has been on and stays on.
My mechanic keeps clearing it and washed the area under the hood.
The car runs fine: no foul smelling exhaust.
Still, I decided to go to get a diagnostic -at a local dealership- to diagnose why the engine light stays on.
Result: the cat converter needs replacing.
In Mexico, the quality of regular gasoline is poor, so I use Premium and always add a high-end fuel injection cleaner with each tank load.
Should I really start shopping for a cat converter, or just replace the O2 sensors? One was replaced 2 years ago -the upstream one- Any suggestions - I would be really grateful. 
How about getting the actuarial code - Like [P1234] and post it back here. There is no code that says the converter is back, there are codes that indicate there may be a problem relating to the converter. Don’t start off replacing a converter when it may not be the problem or it may have been damaged by something else that is causing the problem.
I don’t think the O2 sensors are pissed off at you because you bypassed them. But since you have once bypassed them, can you do it again? If your catalytic is bad, it is the rear O2 sensor that is reporting it. It’s a toss up, bad sensor or bad catalytic. In truth, the catalytic converter is a dinosaur, a mostly useless part that was once a crucial first step in the reduction of hydrocarbon emissions (not to combat global warming, but to reduce ground level ozone, and smog). It’s a relic now, and if it fails it can only do so as a result of some type of engine failure.
Typically this would happen if your engine for some reason ran very rich at some point, to the point that the plugs were fouled by gas and black was pouring out of the tailpipe.
The failure of a head-gasket, which you say you replaced, might cause your catalytic converter to fill with water vapor and fail, at least for a while. But of course it would also dry out very quickly.
You say you replaced the old plugs as well. What was their condition?
You probably damaged your car when you installed and ran that POS “hydrogen fuel efficiency unit” snake oil in your car, and disabling the O2 sensors.
I hope after you disabled that POS, you returned the car back to the way it was with the O2 sensors.
Chances are, you probably need to replace the converters.
BC.
What is a “hydrogen fuel efficiency unit?” Is there a web site that I can look at one? I would like to see this miracle of modern technology.
I live in Baja, Mexico now…
So ignore the light and the converter. What’s the problem?? Oh, you still have Cali plates on the car and must take it home once a year to get it smogged and buy new tags, something like that?
What makes you think Pemex Premium is any better than Pemex Magna?? Half the time it’s the same fuel…My Crown Vic runs fine on Magna…At least it’s 100% gasoline…
This is all BS…You can’t “bypass” an oxygen sensor. If you disconnect it, you get an immediate CEL…
You can “spoof” one by making a resistance bridge and providing it with a power source so the ECM THINKS there is a oxygen sensor, but this is a tricky operation and very few people have the skills to do it…
Thanks to all of you who gave great insight into my dilemma! I really appreciate your input. I tried to get the error code from the tech: he did not save it, but they insisted that it was not the O2 sensors, or anything else, only the ‘catalizadoro’ has gone bad, and here in Mexico they routinely get rid of them once they start acting up. So, for 20 bucks they will remove it tomorrow…
Oh, yeah, the Hydrogen unit worked fine saving me about 30% mpg, clean exhaust (water vapor), but it kept on blowing the safety fuse we attached to it… so we disconnected it. If you want to learn about it, Google words like ‘how to run your car on water, etc…’. The guys who installed it are now busy making generators to power up a home based on this Hydrogen technology, and no longer make Hydrogen converters for cars…they have more need and more orders for those… Best of Luck, Everyone! 
Yeh, they recommended the premium when I first arrived here, did not know better… the car was hesitating uphill, and I was told, it is the poor quality gas… kept adding fuel inj. cleaners, too, I will stop that now…not sure about the tags yet…
that’s EXACTLY what we did, I think it was called EFE - we got it from someone on the net…