I have a 2003 ssei. Im having a problem with the converter. I keep getting a low efficiency code for my car. I have replaced the converter with a magnaflow converter, and replaced both oxygen sensors. And it keeps coming back.
Notes: I am a journeyman mechanic, and I am stumpped. lol. I need some new ideas on what could be causing this. The car is stock. It has all new gm plugs and wires, injectors cleaned, new air and fuel filters, no vacuum leaks, new o2'*, new converter, cleaned the MAF sensor. I reprogrammed the computer with the newest gm updates. Still the service engine light comes on. Another note.. My car has 280,000kms on it.

It also sometimes has a hesitation when first started (like 1 or 2 times a week). It hessitates and bogs when you rev it up, and sounds like a really bad vacuum leak coming from the throttle body but it only lasts a minute, then goes away, and runs fine. (i've read the live data, and there is no missfire data, and from what i can see all the sensors are working) I've been leaning towards a bad MAF sensor, but it tests good everytime. No codes other than the converter code. Any ideas would be greatly appreciated.

You may need to replace the converter. Some of the Magnaflow converter will throw out ODBII codes. You may want to call Magnaflow and ask for help I have heard that the converters that have 4" and 3" wafers will prevent most ODBII code throws.

I had this code before i changed the converter, and i saw nothing wrong with the stock converter that was on the car.
I will contact them tomorrow and see what they say

The problem is most likely that you replaced with a magnaflow converter. The magnaflows seem to be fine in OBDI and 96-99 applications for the most part. However as the PCM'* are continually upgraded the 2000+ vehicles do not appear to like the Magnaflow cat from an efficiency standpoint.

This has also been noticed in a few instances like Canada where tailpipe is done all the time.

So what can i do to solve my problem. I dont mind changing the cat again, but i cant afford to pay $1100.00 for a new gm converter. Ive also been thinking about an intence racing pcm. could they make it so the light doesnt come back on?

a cat sold from a muffler shop should be fine. paying 1000 vs 150 ftw. gm and all dealers rip people a new one with oem cats. i would contact magnaflow and see if they will refund you the money.

correct me if i'm wrong but the 02 picking this one up is behind the cat right? why not just get an 02 sim for 15 bucks and be done with it...

I just had Goerlich 608227 cat put in my 2000 SSEi. No engine lights. 3" Goerlich Catalytic Converter OBDII 608227 - 608227

I have a magnaflow cat from Hogan Peformance and it also set the code (P420 I believe). If you get a reprogrammed PCM, they can program that code to not report. No need for a new convertor.

I talked to intence racing and there going to build me a pcm. And im going to get my money back from the place that sold me the cat.

Ok, I'm looking at dates on this thread, seems to have lost interest. So, I'm going to risk being spanked and do a little hijacking.

I've read elsewhere and here that Magnaflow Catalytic Converter is a great replacement. But then I also read that on some of the newer cars the Mag Cat will cause a code to be set. BILLBOOST37 said something about 'efficiency'. Is that EMISSIONS CONVERTING efficiency or EXHAUST FLOW efficiency?

I ask this because a few years back I had the CAT replaced on my 92 SSEi. The supercharged version of the Bonny uses a high flow CAT compared to the NA bonnies. I went into sticker shock over the dealer install price so I sought out a local repair shop. The guy at the local repair shop talked me into putting on a smaller generic brand CAT on the car. When I questioned him about the differences the guy got irate for questioning his technical knowledge, skills, and expertise. Yeah, I know what you're thinking, me too, but I was in a hurry, needed my emissions done ASAP, so I gave in.
Well, I didn't take very long for me to feel like I had just been scr$%ed. The car seemed to have lost power. I speculate it'* because the replacement CAT just doesn't have the thru-flow characteristics needed for the SC engine.

Since the car needs a new muffler, I ask my fellow forum brothers and sisters, what'* the best bang for the buck on CAT and muffler for a Supercharged 92 Bonny?

Thanks, JB