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Z3 catalyst efficiency nightmare

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So I bought the car with the check engine light on. The owner said its been on for a year or so. Its always been the P0420 code.

About a week ago i decided to fix the issue. Some of you may know I had issues with getting a good scan done..

Anyways I have been using a Peake FCXII it pulls up fault table 07 #28 has been the code.

I replaced the rear O2 =code came back
I replaced the Cat (weld in from muffler shop) =Code came back

Whats left the front O2?
I cant seem to find any Vacc leaks and its running good.

I really am surprised that I am not pulling up anymore codes espically if its a big enough issue to through a catalyst efficiency fault.

Please any info/ideas would be great.
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The catalyst monitor compared the switching rate of the post-cat O2 to that of the pre-cat O2. The switching rate of the post-cat should be about half that of the pre-cat O2. It is hard to tell without looking at on a scope or sometimes you can get a decent reading from a professional scan tool that graphs data. A pre-cat O2 will typically set a slow response code if bad. I have never seen a pre-cat O2 set an efficiency code to date, though I am not saying it is not possible. 3 things involved in setting code, Cat - O2 sensors - and Computer. How quickly does the code return once cleared? Was a good quality cat installed with no leaking welds? If I recall, yours was a Z3. Is there one cat or 2? Was the proper one replaced if there are 2? Typically, efficiency codes take a while to set. If code returns quickly, most likely wiring or computer problem. Look at the wiring of the post-cat O2, especially where it routes around the exhaust for chafing or burning. Also look at the connector for signs of fluid intrusion. Oil leaks that get inside electrical connectors can do strange things also.
It is a 1.9l Z3

It takes about 70-80 miles for the code to return

I watched the guy weld it up and it looked good, he did check for leaks after it was all done.

O2 wires look properly routed when again i watched him reinstall the O2 sensor after the cat was installed

here are the specs for the cat
***8226;Inlet / Outlet: 2.25" Offset / Center
***8226;Max Engine : 5.9L
***8226;Test Weight: 6250 lbs
***8226;O2 Port: No

I am going to check the wiring of the front O2 today, and possibly replace it since its only $65.
Maybe this link helps: http://www.ronstultz.com/knowledge/Documents/Aftermarket Catalytic Converter - A Warning.htm

If everything is ok and well installed, it seems like you have two options:
1) getting the OEM catalytitc converter
2) You could keep the replacement cat, but you need an O2 simulator in order to turn off the P0420.
Many have not had good luck with non-OE cats. They're in the same category as the cam sensors and mass air flow meters. Seems to be best just to buy OE for those.

Did you confirm that your old cat was bad?
You can put the pre-cat O2 in, but I do not believe it will solve your problem. It is one of the main fuel metering sensors in the system, and is probably a little sluggish due to age and mileage. Looking at the link juancarlosmgb included, I believe your problem is going to be the cat. The cat in the photo sets a good distance away from the manifolds. The cat has to heat up to light the catalyst. The inferior materials in the aftermarket cats may have difficulty lighting off that distance from the engine, especially in the time frame the computer is looking for it to do so. If you notice on most newer vehicles, the cat is extremely close to the engine, if not bolted directly or incorporated into the exhaust manifolds. This is done to my understanding to light the convertors faster and to meet the stricter CA standards. I know it's not what you want to hear, but I believe that the cat is going to be the issue here.

You might try a salvage yard. Greenleaf is on Hwy 78 between Athens and Monroe. They are owned by LKQ and that particular yard processes European only. They may have or can transfer a cat for you. I also believe there is a BMW specialty salvage yard past Winder on the way to Road Atlanta (Hwy 53 I believe). No experience with them, I just know I pass by there occasionally.

Good Luck
Maybe this link helps: http://www.ronstultz.com/knowledge/Documents/Aftermarket Catalytic Converter - A Warning.htm

If everything is ok and well installed, it seems like you have two options:
1) getting the OEM catalytitc converter
2) You could keep the replacement cat, but you need an O2 simulator in order to turn off the P0420.
Thanks for all the replys. I read the write up about factory converters and it makes sense. On my car they installed the new cat within inches of the factory cat position. Hopfully this is good enough for my car.

Yesterday I replaced the front O2 and "knock on wood" the code hasnt return. Ive driven a total of 115 miles with several short and long drives. And a complete cool down. I think I may be in the clear.

here was my front O2. It looks rough
futuresvt : Glad to know that you solved your problem!!!
Let us know if the code does or does not return. I am very interested to know the outcome on this. I suppose with the new pre-cat O2 sensor, the switching rate could be faster and therefore meet the monitoring criteria. Let us know if it does come on the approx. mileage and if not just keep us up to date. I would appreciate it for future reference.
Thanks for all the replys. I read the write up about factory converters and it makes sense. On my car they installed the new cat within inches of the factory cat position. Hopfully this is good enough for my car.

Yesterday I replaced the front O2 and "knock on wood" the code hasnt return. Ive driven a total of 115 miles with several short and long drives. And a complete cool down. I think I may be in the clear.

here was my front O2. It looks rough
Update: Its been over 500 miles and NO check engine light. Thanks again for everyones help. This has been my first z3 and bmw actually. But i have just sold the car and I am moving onto something else. Although it was a fun car and i enjoyed it. Its going to be awhile before i step back into another bmw lol.
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