Bingo! Thanks to your description vs the available DTC codes it looks like your '98 VCM is kicking a
P0430.
Note: For comparison, on a GMT400 a weak
left cat would cause a P04
20. (Bank
1) Your code is for the
right cat. (Bank
2)
1) The short answer is that the computer is monitoring/comparing the activity of the forward O2 sensor
ahead of the right cat versus
the activity shown by the O2 sensor
behind the cat, and essentially once the rear sensor's behavior starts to mimic the front sensor the
VCM illuminates the SES light and stores this code. (These 2 sensors on a weak/bad cat will start to act like they are installed on a
straight piece of pipe.)
2) The rule of thumb is that the car owner has to replace the cat in order to clear the code. While it's true that cat replacement will
clear the code, IF there is
still an underlying issue upstream of the new cat that led to the overheat/failure of the original cat, then
the new Federal $$$ (or even stricter Cali emissions $$$$) cat may prematurely fail and need to be replaced again. (!) In order to
help you prevent this expensive repeat scenario from happening to you, I'd like to expand on this a bit.
In a river, pollution flows downstream.
On our 5.7 engines, cylinders #1,#3,#5,#7 flow into the downstream left (Bank 1) cat.
And cylinders #2,#4,#6,#8 flow into the downstream right (Bank 2) cat.
IF both left and right cats failed, then either you have a healthy motor and the cats just aged out/reached the end of their service life
around the same time. Or, it could have been a combo of a global error in the VCM<> engine closed loop that caused the premature
poisoning of both good cats.
I bring this up because you have a DTC-free left cat, but at the same time you also have a P0430 right cat.
This would lead me to carefully compare & contrast the overall engine health of Bank 1 vs the overall health
of Bank 2. The first step would be to pull all 8 plugs and see if you have 1 or more bad plugs in Bank 2.
(See attached for arranging the plugs for a group photo for the
easiest most conclusive remote analysis.)
For what it's worth, a single misfiring cylinder in one bank can make that bank's cat runner hotter/overheat/weaken
sooner than if all the cylinders in a bank are firing properly.)
In English, if the plugs/wires/cap/rotor have been in service for any length of time, I'd seriously consider first performing
a full tune up. Once the engine is running as clean as possible, if it was mine I would try some Cataclean on it, following
the directions carefully. Since this DTC was flashing recently before finally coming on solid, even though the cat is
failing the efficiency test, with a clean engine and the catalyst cleaner, you may able to get the cat to come back just to
the good side of the failure criteria. (If a new cat = 10, and an inop one =1, the test failure may indicate that you just
went below 3.)
If the tuneup & Cataclean can get the cat back up to a 4-5 on this scale, then the light goes out, and you can continue
to run your original, revived cat indefinitely. (Usually this is in a high mileage DD where passing the next yearly state safety
inspection is contingent on several other nearly worn-out subsystems. So the mindset becomes why just swap in a $$$ cat
on a car that might encounter a real showstopper elsewhere within the next 12 months? Maybe I can bring the existing cat
back to life for a little while longer?
And even if you are too late to revive this cat, then with the fresh tune up the new cat will have the easiest possible life.
And if you discover an engine problem (bank 2 head gasket, dribbly injector, bad spark plug wire, etc) during the
tune up effort, then at least you know to fix your engine before replacing the right cat...to prevent the new one from
failing prematurely. (!)
3) Taking the P0430 from the shade-tree mechanic/common sense level to the pro mechanic/emissions-lab perspective,
here's a good article to noodle on: (
P0420 P0430 vehicleservicepros article)
****
So there you have it. To recap:
Conventional wisdom is DTC occurs, immediately find a part and swap part out.
Cleaning up the engine as much as possible with a fresh tuneup first is drawn from personal experience.
Have helped myself and others by cleaning up the way an engine runs & making life easier on the cat...to
the point where it recovers enough to get back on the good side of the P0420/0430 testing pass/fail criteria.
Sometimes the (previously overloaded) cat comes back on it's own. But once in awhile we'll try the
cataclean stuff and this seems to help in the tougher cases. (Of course, if the cat rattles when you
shake it...it's been too hot, the honeycomb has melted, and you are definitely too late to do anything else
but replace the wounded warrior.)
The last part is a pointer to the theory & graphs that I like to share on the back of a napkin at cocktail
parties. The ladies swoon and the mechanics weep with joy when I do this. :0)
****
That's all I got for now. If you could report back with what you found and how you finally
fixed it, then this would be a huge assist for others that are researching similar issues.
Best of luck. And welcome to the GMT400 forum!
Cheers --
