The Dreaded SES P0430 Code

Hi. I’m new to Car Talk and I’m hoping for some good advice or possible solutions: Sorry for the length of this post, but you need to know the facts. I recently purchased a used, high mileage 2004 Infiniti G35x (AWD) 145K miles from a reputable new car dealer here in my town. The car was impeccably maintained and clean inside and out. Just a week or two after the purchase my SES light came on revealing engine Code: P0430 (catalyst efficiency below threshold bank 2) pointing to a new catalytic converter in my future. Infiniti recommends premium unleaded for this vehicle. I found out my son filled her up with mid-grade unleaded. Is there any chance this could have triggered the SES light to come on? (Previous owner states he always used premium unleaded in the vehicle since ownership.) I also double checked the gas cap to make sure it was on tight. I find it extremely odd that the SES light would come on so shortly after the purchase. I mean what are the odds? Don’t dealers run a diagnostic scan of their trade-ins to check history and for potential problems prior to selling their used cars? I test drove this car numerous times without any indication of a problem and right on the heels of the purchase it passed state emissions… now just a few days later, the SES light comes on. Why wouldn’t emissions pick that up? I have reset the SES light (code) and filled her up with premium unleaded. Now it’s a waiting game to see if the light comes on again. If the light reappears, I plan on taking it back to the dealer to work out some kind of “reasonable solution” to repair costs. I’ll find out just how willing he is to stand behind his cars. I realize that this is a high mileage car, and I hardly expect perfection, but I didn’t see this coming. What to do… what to do… By the way, I checked all the fluid levels … no discoloration in the oil (fresh, golden brown)… no discoloration in the coolant… transmission fluid, good… all other levels are good. Car runs fine, no rough idle in cold… Thanks for your advice / comments

You’ll probably get mad at me, but I am going to translate your logic about what “should have been known” (or picked up or pre-diagnosed or whatever) into another issue. Last summer I had a set of new tires put on my van. Two weeks later one of those tires went flat. Shouldn’t the shop have known when they put the tires on that one was going to get punctured and go flat within two weeks? Your logic is no different.

First, you don’t even know yet that you have a problem. Clear the code and drive on.

Second, even if you have a P0430 code that doesn’t mean that you need a catalytic converter. The code is a starting point for diagnosis - not a diagnosis itself. All it means is that one of the rear O2 sensors isn’t sending readings back to the car’s computer that are what they are supposed to be. One possible reason for these readings is that the converter isn’t doing its job.

Third, your engine could throw a rod tomorrow - as could mine, or anyone else’s. The radiator could split open, overheat the car, and wreck the head. A transmission line could blow and fry your transmission. Any of these things could happen without having any way to know it beforehand. The knowledge that it is possible to get about a car’s condition is limited. Literally anything could happen next.

Thanks, I appreciate your straight forward comments… I’ll keep my fingers crossed that the light doesn’t come back on.

The octane of the fuel has nothing to do with it, the mileage on the car has everything to do with it… Infinity’s wear out just like everything else…In the not to distant past, a new car dealer would never put a car with 145K miles on his lot…That trade-in would have been auctioned…

Oxygen sensors (that’s the part that triggered the P0430 code) have a design life of 100K miles. So I would change that part first should the CEL come back on…

Thanks for responding.

On many, perhaps most cars, there is no way to check output efficiency of the cat convertor. The sensors measuree the exhaust before the convertor, then after the converter. If there is no difference after warm-up, it makes the simple decision that the converter is not working well.

So, if the sensors are off from spec, it may call a bad converter, when it is a bad sensor.

I am not a mechanic, but when I had a problem (which went away by itself) I Googled, and a number of mechanics said they almost never had a bad converter, it was almost always a bad sensor. I can neither confirm nor deny that statement.

It could be caused by an exhaust leak as well. Make sure that’s not the problem.

O2 sensors can be a pain.
The computer measures the one on both sides of the catalytic converter and the ratio has to be a certain number. If it isn’t, it throws an efficiency code. If you have this diagnosed at a dealer, they’ll replace both sensors and the cat because they don’t want to see you back and make the most money - an expensive thing for you to do.

If you happen to know someone with a scan tool, ask him to measure the reading from the O2 sensors and make sure the voltages vary with engine speed and they are roughly within limits (car manuals show what those limits should be).

Could the regular fuel and consequent retarded timing put this geriatric converter off its game?

No…The air-fuel ratio stays constant, controlled by the front o-2 sensor…The converter sees the same exhaust gas levels…

RemcoW, I’m kind of curious which scan tool you use to read the voltage on the oxygen sensors?

I have two: an Equus Innova 3140 and a CarChip. The Equus is great for hooking up and watching levels and stuff while driving. The Carchip is nice because it records 300 hours of parameters and time stamps them so you can look back and see what caused what when. It hooks into a PC to read out so that’s more a tool I used to find intermittent problems.

I have a simple USB to OBD-2 cable and a free-ware computer program and with a lap-top it will read in real time the entire operating parameter of a modern car…