Car - 2001 Hyundai Elantra, 2.0L, 5-speed manual, 63700 miles.

I bought this car with a P0422 (Main Catalytic converter below efficiency threshold). My state doesn't have emission laws. The upstream O2 sensor was fluctuating always, and the downstream frequently (about once 3 or 4 seconds) dipped below 100 mV. This seemed like the cat was genuinely screwed. I found the same car in the junkyard and replaced the cat, and the o2 sensors from the other car. The error code did not reappear.

This lasted for about a month. The downstream O2 sensor was also steady at 700 mV after warming up. Yesterday, MIL latched on again, and it was P0422 again. This time I made the following observations with the downstream O2 sensor -

1. During idle, it maintains around 700 mV and occasionally dips (about once in 10 secs).

2. If I VERY slowly increase throttle, it is happy and maintains 700 mV.

3. But if I push a little faster (practical for a real driving situation), it dips and recovers.

So I suspect that the problem is elsewhere, somewhere above the cat, in the fuel ratio. I don't know where to start, or if my suspicion is right.

There's always the question that the replaced cat was inherently bad, it coming from the junkyard. But that probability is less I guess, and given the fact that it was working well for a month. I drove a lot of city and highway during the month, so if it was bad in the first place, ECU would have caught it sooner.

Another observation that might help - sometimes (so random that I cannot qualify further) when I press the clutch (with 0 throttle), the engine rpm shoots up to 3000, lingers around there for a few seconds, and then comes down to normal idle.

Any ideas?

Thanks.

What is it that your hoping to see in the downstream sensor readings?




78 Corvette Pace Car For Sale-18k mi orig-Florida

A steadier reading maybe? In the old catalytic converter, it was never steady, fluctuating between 15 and 700 mV always. So I'm guessing that the same will happen to this converter as well, eventually. Please correct me if I'm wrong.

A lot depends on the temperature of the Cat which is where the RPM factor comes in. A constant .700 is not real good. What you don't want to see is the rear sensor mirroring the front sensor. It really shouldn't be cross counting at all. A steady reading below 500 would probably be ideal. The computer knows what to look for before setting the code and it has to see it more than once. I certainly don't have a lot of faith in used converters. That's against Federal law too by the way.




78 Corvette Pace Car For Sale-18k mi orig-Florida




(This post was edited by Hammer Time on Oct 18, 2009, 12:39 PM)

Here’s my thinking/guess; your original converter was burnt out for whatever reason (likely over fueling or misfire). You replace it with a used one, that was on its last legs and whatever burnt out the original, took this one out as well.

Now, you don’t have to guess whether the converter is bad or not because they can be tested; with the use of a gas analyzer and this math formula.
Pre cat gas minus post cat gas divided by pre cat gas multiplied by 100 equals catalyst efficiency percentage.
Anything below 70% efficient is considered junk.

At the same time the converter is being tested you can determine whether the engine output gases are within acceptable levels; as to not burn out the next converter.

Converters do not die of natural causes; they are murdered.

Acceptable pre converter levels are approximately .5% (1/2) carbon monoxide (CO), 50 parts per million hydrocarbons (HC). As for the other gases O2, CO2 and NOX; well they’ll mostly fall in place, once the CO and HC are with in spec and it is high levels of CO & HC that take out converters.

I know that a DIYer will not likely have access to a gas analyzer but I only charge about ½ hour’s labour to perform the test.
This entails drilling a small test port in front of the converter; taking my gas analyzer samples and then plugging the test port; the post cat samples can be taken at the tail pipe.
I do a fair amount of testing on converters; being an accredited repair facility in Ontario’s emissions testing program. Almost every time I find a bad converter; it’s because the engine wasn’t running properly or there has been some form of engine management work done on the vehicle, very recently.

Dan

Canadian "EH"

Thanks guys, for your views.

I actually was debating about buying a new cat, but the junkyard cat came by and I decided to test it before installing a new one. Looks like what I thought is right, and it died.

If something did kill the cat, where do I start to look? I'll try to get the gases analyzed as soon as possible. But if I do find unacceptable upstream gas levels, what's next?

Thanks again.

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That all depends on what combination gases are out of line; so to answer your question, I don’t know, not without testing things or receiving accurate test results.
High CO is a product of incomplete combustion of the air fuel mixture. Where all the oxygen has been burnt away; with no oxygen there’s nothing to feed the fire to burn off the remaining partially burnt carbon.
HC is a product of no combustion of all or a portion of the air fuel mixture in the cylinder; in other words raw un-burnt fuel, leaving the cylinder.
There is not a go to thing that will cause higher then “normal” gas readings; but through testing of electronics, ignition & fuel systems and not forgetting that there’s a mechanical engine under all of these systems; you can eliminate the things that are not at fault.

Look at it this way the actual gas readings are like the trunk of a big old tree; at the top of that trunk are hundreds of different branches. You decide on which branch to clime out on, after you’ve performed tests and analyzed those test results. Believe me its not hard to be way out on one of those limbs and then find out you’re wrong.


Dan.

Canadian "EH"

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LOL........How true it is




78 Corvette Pace Car For Sale-18k mi orig-Florida