'99 OBS - CEL - cat efficiency

[rick]

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

 

[Edward Hayes]

The bad CAT converter signal comes from the O2 sensors. Subaru had a recall
on O2 sensors about 1999-2001 and although you are out of warranty I suspect
the O2 sensors. Get another code reading from another dealer or shop. My
AutoZone will do this for free if (I think) you buy the parts from them.
eddie

 

[Ken Gilbert]

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

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:
http://ken-gilbert.com/cars.htm

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

 

[Dominic Richens]

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.

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.