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P0420 OBD2 with OBD1 Engine???
Hi guys this is my first post.

Long story short, bought a car from this Honda Service parts manager and car was great after I test drove. Go to pick up car then next day and I'm driving home and the check engine light comes on. I go to Autozone and they check is and the code is P0420 which is catalyst system efficiency below threshold bank 1. I text the owner and he acts stupid about the whole thing saying it was fine when he had the car and what not. I'm guessing he cleared the code...

Okay point is I was looking into replacing the catalytic converter but the previous owner put in a OBD1 D15b on the car instead of the OBD2 D16Y8. Should that be a problem? I know they're two different computers, will I have problem passing smog next year if I replace the cat and the light still doesn't turn off? (He already passed the car). He just swapped the engine and tranny, he said he kept the original exhaust system on the car. Really lost and feel like an idiot for not driving the car longer to see that light pop off. I had the cat recently checked by a mechanic and he said it's at proper temperature, he said it could be the OBD1 engine with a OBD2 ecu giving off the check engine light. The OBD2 is looking for more things that the OBD1 is giving out. It's a 2000 Honda Civic EX of course lol. Car drives fine with no problems, just that annoying check engine light bothering me. Sorry for the long post. The ECU I have is P2P A-12.
Either an issue with cat or the o2 sensor. If the car is running correctly otherwise then the engine is setup for the obd2 ecu and shouldn't be your issue. The cat being at a certain temp range doesn't mean it's working correctly.
Sorry for the late reply, tried going on Honda tech for the answer and had no luck. Il definitely check that out! Could I still pass smog even f I were to fix that or is the different engine gonna affect it?
Okay, this is a bit of a hodgepodge so I'm gonna just delete my whole previous response!

So the guy put a D15 motor and ecu in a 2000 vehicle, do you happen to know what D15 motor he used? Seeing as it is using an OBD1 ecu, that tells me that it is older than the car itself. That is illegal, you cannot put an older motor into a vehicle manufactured after the motor was made.

That said, I wouldn't be surprised if it happens all the time, especially in the Honda community. It's also probably something that can only be detected visually (and by someone that knows what to look for.) Not important to your issue, but it's just good info for you to know.

As for running a D15 (OBD1) motor with an OBD2 ecu, I'm going to have to side with your mechanic. As cars have evolved through the years, so has the On Board Diagnostics (ecu's) ability to fine tune fuel, air, and spark in order to maximize performance, fuel economy, and low emissions. That means that OBD2 has more sensors everywhere, and it cannot find them on an OBD1 engine! The knock sensor immediately comes to mind, and I wouldn't be surprised if there's another O2 sensor that isn't there either!

You either need to get a OBD1 ecu and see if that "dumbs" the car down to where it doesn't notice what's missing, or perhaps look into doing a legal swap someday?

What is a bugger to me is how he managed to pass smog. They should have immediately detected this as soon as they tested it.
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He put in a d15b and the p2p 12 ecu is in there as well. Like I said the car drives fine. He works at Honda, ironically and he reset the check engine light and I drove it around to set the ecu and he passed it. The light didn't come on. But it came on two days later Guessing it was an illegal pass. But he passed it only because that light came on and I came back to him. But like the other person said maybe it's the catalytic converter or an O2 sensor. I'm getting that checked out soon. Will keep updated. Thanks for the replies guys.

He also included with the car a p2j-003 ecu 3 stage vtec.

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