Rich O'Rourke

Hi, I replaced the post catalytic coverter oxygen sensor with a delco 130 OEM . The P0420 code still comes up when I use my code reader. It has been 5 start and drive cycles of at least 8 miles each. When i put my code reader on the screen looks like this:
On board Monitor test
TEST $OC
ID 60
MOD $10
Meas 81D8
Max 8000
Min -------
STS Fail


What does that mean? What part of the system is failing? What part of the exhaust system corresponds to the $OC?
I do not know what part is failing!!
Thanks, for any help at all!!!!
This is a 2001 chevy cavalier,4 cylinder, manual stick shift,2 door coupe with 85000 miles on it. 1)New spark plug wires
2)New GM plugs platinum
3) Both coils new that go to the spark plugs
4) New collant and thermostat
5) New temp sensor on the engine block
6) New pressure valve that goes on top of the engine near the fuel injectors
7)New Boshe oxygen sensor in front of the cat.
8) New cat from national Auto. The P0420 code started when I had 540 miles on the new cat.

Thanks many times over for any help you can supply.

MDTAHOE

Replace the cat with a better one. 99.9% it is a bad cat. More than half of the aftermarket cats do not work for very long if they work at all.
I see you posted once before and I told you the same thing so I guess you really do not want help. I do check engine lights everyday. This is not my first day. I guess you can keep throwing parts at it.

Rich O'Rourke

HI, yes i do want help with this, and I listen to what everybody has to say, i did not replace the cat yet since the parts manager where I purchased it insisted that the National Auto cats never are defective,also one of your compatriots mentioned that the oxygen sensor after the cat is supposed to be very lazy in response and since i had replaced it with a boshe, which has too quick a response,,to put a delco sensor in. It is far less expensive to try an Oxygen sensor than to come up with 900 dollars for a GM cat from a chevy dealer. Replacing the cat was next on my agenda. So...since you are the professional mechanic who has seen these problems many times before...any suggestions where I could get a reliable cat on the internet? What brand would be a good one? Also..I was very curious what the script notations the code reader listed said.
TEST $OC
ID 60
MOD $10
Meas 81D8
Max 8000
Min -------
STS Fail

I appreciate your opinion and intend to get my cat tested and go back to National Auto for a refund. Out of curiosity...can you tell me what the code reader messages signify?
Thanks
Rich

MDTAHOE

I have a $10,000 Snap -On Verus scan tool and never seen that.

Here is how PO420 is generated the computer looks at the o2 before the cat and after the cat in millivolts. Below 500mv is lean above is rich. The voltages should never match the mixture coming out of the cat should be almost always below 500mv (lean) if it is not it means the cat did not clean the mixture thus a cat efficiency code. Watch the data on both o2 sensors if they pretty much mirror each other the cat is dead. Cat must be at operating temp. You can also find a shop that has a 4 or 5 gas analyzer, have them perform a oxygen capture test or "snap test". When a converter completes the catalyst function it should use all the oxygen in the cat. They can take a base oxygen reading then snap the throttle is the o2 reading goes up over 1.2% the cat is dead. At my shop we have a 5 gas analyzer so I use this test as a double check on all PO420 codes and have yet to find one that passes.

Here are some links that may help

https://www.carchex.com/goss/article...rter-confusion

http://buy1.snapon.com/catalog/tools...e=snapon-store

http://www.underhoodservice.com/Arti...ic_update.aspx