I'm sorry for bringing this old thread up, but it was the closest one to what I'm looking for and I did not want to start a new one.
I just learned a very important piece of information here, i.e. OEM CAT's (around $1100) is not much better than an after market one ($300 direct fit or $75 universal) and that's a lot of saving for me. I still do have a few questions though and I will put them in points below (easier to read than boring paragraphs, I guess).
1. Car I'm concerned about is an 08 grand prix with a V6 3800 series III engine that has about 158KMiles that had O2/CAT codes a couple of times last year that when deleted did not come back till now where for the last couple of weeks, PO420 comes back almost the next day whenever I delete it. The code means that CAT efficiency in "Bank One" is not good. I did some research and found that there is another code called PO430 for "Bank Two". My first question is, if my big V6 engine only has one upstream and one downstream sensor and one CAT after which the tube splits into two and each ends in two openings at either side of the rear (simply by looking under the car), then are there even bigger systems that utilize the PO430? Do they have two separate CAT's for the different banks of the engine and 4 sensors?
2. I used a Bluetooth OBD2 gizmo with my phone attached to the dash displaying the wave-forms of the upstream and downstream O2 sensors simultaneously for many days staring at those wave-forms as much as safely possible while driving. The upstream is oscillating as it should but it looks closer to random than I thought. The amplitude may range from 0.1-0.6 or 0.3-0.8 or even sometimes missing a bounce up or down and it looked like pressing the pedal increases the peak-to-peak bounce to more like 0.1-o.9V. The downstream in more or less constant at close to 0.7V but sometimes (rarely) does bounce but with a much lower overall change than the upstream. They both start at about 0.45V bias when starting from a cold engine, but there is a random point in time that triggers the code when the engine warms up after driving for a while where the upstream signal keeps bouncing but at about 4 times slower and at a wide range (probably 0.1-0.9V) while the downstream shows almost the same shape but a tiny little bit lagging. That code triggering behavior only lasts for less than 5 seconds and the waveforms go back to normal (so if i delete the code, it may never happen again for hours). Does my description show any sign of something wrong other than the CAT?
3. I read here that CAT's last about 150K miles (I'm sure that the writer has that theory from a long experience and that it is a rough estimate that could be more or less), and that the car felt like new after replacing it, and then a local NAPA guy told me yesterday that it is a common practice to replace both sensors when replacing the CAT because the almost always age together. Considering the age of this car, is it crazy if I just go ahead and replace all of them without real and extensive diagnosis (that may cost more)? By "extensive testing", I mean, some of the crazy things that people with $10K snap-on scanners do, like propane test for O2 sensor reactions, or smoke machine for vacuum leaks or advanced O2 sensor wiring tracing.
4. OEM or cheap? Direct-fit or universal?
note: both O2 sensors are $32, while AC-Delco (made in China) is $120.
Is this whole job a DIY-friendly one? and does installing universal cost a lot more than direct-fit?
It is an aging beauty, so spending a buck less on her is a great thing for my current financial situation, and any help by advice or opinion is greatly appreciated.
