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'99 OBS - CEL - cat efficiency

I have a '99 OBS (California emmissions) which I purchased used 1.5 years ago. 76k miles when I got it, now has 107k. I've had about 5 or 6 instances where the CEL came on and the code is something like "cat efficiency below threshhold - bank 1". The first time it happened, my mechanic said, "You're using cheap 87 octane gas, aren't you?" I was. He said he's had something like 17 people come in with that code, and in all but one case, switching to high-octane, brand-name gas solved it. The one case required a new catalytic converter. Well, after the first time I've been using Exxon 93 octane (I cheated a coupla times) and I still occasionally get the same code....seems to be more frequent now.

So before I start socking money away for a new cat system......

1) Is the gas really the culprit?

2) If the codes are coming from the cat (after the fact), wouldn't that mean that the problem is really in the engine/combustion process, and putting a new cat on is basically like "emptying the garbage can, but continuing to make garbage"?

3) Could the O2's have anything to do with it? I've asked the mechanic repeatedly about the O2's, ie can they be bad/dirty and not send a code to the ECM? He says, "no".

4) Could the cat sensors be bad/dirty? Can they be replaced without replacing the entire cat?

5) Is the problem caused by something else entirely?

TIA, Rick

Reply to
rick
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Reply to
Edward Hayes

hey rick,

a cat inefficiency code (typically p0420) is commonly thrown on cars which have either a) removed their cats or b) outlived the usefulness of the cat material. since you haven't deliberately done (a) then i must assume that during the course of your 100k+ miles you've reached the end of the effectiveness of the catalytic material.

the way the code is set is that the ECU tracks the lambda (derived from total oxygen content) of each of its o2 sensors very accurately. the front o2s is pre-cat, so it constantly sees a varying lambda as the ECU varys fuel injected, attempting to keep partial throttle closed loop conditions right at stoichiometric ratios. during particular times, the ECU will then compare the readings of the two. with a properly working cat, the rear o2s will show a very slowly changing lambda, since the cat is doing its job by storing and releasing combustion reaction products. if the cat is missing, or non-functional, it will exactly track the fo2s. it would be precisely then that you would throw a code. it does not CONINUALLY test the cat condition, which can lead to it being thrown at very strange and seemingly unrelated times.

if there was a problem with the actual o2s, either one, then you would get an entirely different code--and it would prevent the cats from being "tested" by the above algorithm, thus preventing the p0420--so that's not likely to be the case.

i've got some more information pertaining to OBD2 here on my website:

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as for the gasoline rating causing it, it may create conditions in which the cats DO work more or less efficiently, but clearly the current cats are marginal and should be replaced. if you expect to pass a smog test you'll need 'em.

hth ken

Reply to
Ken Gilbert

More likely that crappy fuel with high sulfer content has "clogged" your cat, and now it is dead. I don't know if there is anything you can do to "revive" it.

-- Dominic Richens | snipped-for-privacy@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!"

Reply to
Dominic Richens

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