04 CEL for cat.
 

Alright guys, I want to remove the ubend on my 04 MC/SC but I'm on a pretty tight budget. Ideally I'd like to get the 3" catless DP from ZZP, but I don't have money for a tune or an O2 sim (which not sure if it would work on my car), so is it easy to trick the PCM into thinking the cat is working with just O2 extenders?

Also from what I've read, the newer cars start running a rich mixture to try and clean out the cat, is that true for the 04 MCs? I can live with the light on my dash until I get a tune but not if it's going to be dumping fuel.

I installed a Magnaflow DP w/high flow cat about 2 1/2 years ago and have not had a CEL pop up since then. The restriction of the cat is negligible and doesn't affect performance.

I'm not too worried about taking a performance hit from a high flow cat, but offroad DPs are cheaper than catted ones. I'm pretty sure my stock cat is clogged up, smells like sulphur after hard driving.
Are there actually any DPs with cats that are in the $100-$150 range?

:rolleyes: Hi MonteMonkey,
I have not seen any in that price range, but maybe one of our member's might know of one (?) Check back on your thread & hopefully a member might be able to help > Good Luck <

p.s. Most of our member's are on a tight budget 4-$ure :(

Smelling sulphur doesn't necessarily mean the cat is clogged. The sulphur content varies at every gas station. The more sulphur in the gas, the smell becomes more evident. Actually, that smell means your cat is actually working.


To verify whether or not the cat is plugged a pressure test needs to be performed.

My understanding was that the rear O2 sensor only measures cat efficiency and has no impact on engine performance, so you shouldn't have to worry about your fuel mileage. Also, from what I've read, people have had success with spacing O2 sensors out of the gas path on high flow cats. I'm not sure I'm overly familiar with people having success with spacing it out on a cat-less setup.

I just installed the ZZP with high flow cat (within the last seven days) and my CEL has stayed off (no tricks needed yet), whereas it had been on for the previous six months. Additionally, since it goes to the heart of what you were asking, for that whole six months I drove with a bad cat, I never saw my fuel mileage take a hit....which would support the rear O2 sensor not impacting fuel scheduling.

As for your $100-$150 comment, I got mine brand new on this forum for about $135+shipping, but there's a reason I didn't change my pipe for around six months....that's about how long it took for me to find one and get it. They pop up for sale every once in a while, but you have to be real quick with your reply to get it.

Also, last comment then I'll stop. My loose understanding if that if you're NA then you want the 2.5" pipe. Part of me says a 3" is only beneficial if you're blown. I think a NA engine actually loses low end torque if you put hte 3" on it, i.e.-your engine really needs the backpressure.

Yeah mines blown, thats why I want the 3". As for the CEL, what year is your car? I've heard that 04 and up start running a rich mixture because its easier on the cats, but that could've only been the grand prixs. Did the PCMs in the Monte Carlos change from 03 to 04 or was it just the grand prixs that changed?

Also thanks for all the help so far, this is really a nice friendly place.

Mine is a NA 2004 SS. Perhaps they changed and it was imperceptible to me. Not sure, but good luck.

I am running no cat and no rear 02. Maybe I am lucky but it throws no codes and runs like a dream. I even gained a MPG or so from going cat-less.

The rear O2 sensor works in conjunction with the front O2 sensor. If the voltage signal of the rear exceeds parameters and gets close to the voltage signal of the front O2 sensor it triggers the CEL. This will affect the performance of the car because the PCM forces the engine to run in open loop mode. Then the PCM calculates the air/fuel mixture off of preset tables in the PCM and off the MAF and coolant readings. The O2 sensors are ignored. Since the engine is running in open loop mode it runs the engine rich and your mileage will drop. Also, the engine will not perform optimal.


You can get a Magnaflow Downpipe/Cat that meets OE specs for around $150.


If you are NA then yes, you should install a 2.5" downpipe.