Jonota

I purchased the car late December w/ about $3500 in "repair" work documented. One of the things documented was a P0420, and the repairs were 2 brand new catalytic converters.
Fast forward a few weeks, and the check engine came on. I ran a scan with the front and rear O2s and found that instead of a secondary O2 following the primary O2 indicating a bad cat, I have one rear O2 staying mostly high, and one rear O2 staying mostly low (seemingly good). I can't remember which is which, but I believe bank 2 is the passenger side, and when I put the car on my lift to check them out, the rear passenger O2 had some carbon fouling around the base, while the drivers side was clean - leading me to believe that for some reason the passenger side is overfueling heavily. (the 19-20 mpg mostly highway driving is also leading me here).
I also noticed that my O2 sensor waves look different. One is a smooth "sine wave" form, the other is of similar outline but is very "jagged" and spikey looking.
Other things that I noticed underneath was that the drivers Y-pipe to header downpipe was loose (but I tightened with no change), and both manifolds to Y-pipe connections instead of having actual gaskets have some sort of RTV on them... gotta love "professional" shop work.
Thoughts? I'm half tempted to just throw some long tubes and an ORY on it w/ new sensors and see how I fare, but that just seems extreme. Thoughts on the O2 sensors? Are the front and rear O2 sensors the same so I can just swap them out?
One attachment is a jpg showing the waveforms, the other is a copy of my 3rd scan on the way home using HPtuners.

Bacon

Not sure about front/rear compatibility on your vehicle, but you can swap both rears to see if the problem stays on one side or switches. That will tell you if you have a bad sensor or a bad air/fuel issue. You could also swap the uppers to check the same. Once you reset the light, you have to drive it for a min of 10 min, reach operating temp, and then sit at idle for a min of 2 minutes (i think). If these conditions are met, the light will stay off even if you have an issue. This is how you can drive thousands of miles and never know you have a bad cat - other than mpg issues.

Jonota

I did swap the rears - the issue stayed on the passenger side which makes me think that the front O2 is not responding properly. When I pulled out the rear O2 on that side it had a carbon layer on the seating ring. I just noticed the difference in waveforms and wondered if the one was too smooth indicating an issue, or the other too jerky indicating an issue.

Bacon

Rears wont change any fueling of the engine, it is just a check for the Cat efficiency. If you had a bad front, it could mess with your fuel trims and cause some issues. Try swapping the fronts, or if they are hard to get to, just replace them. Either way youll want to find the issue quick because a rich running bank will foul your new cat in a hurry. Then youll never get rid of that P0420 code.

P0420 is drivers side, P0421 is passenger.

Jonota

Yes, I know that - that's why I wanted to get some opinions on the waveforms I posted. To me they both seem to be tracking fairly well - the passenger rear is really high giving me the code, but isn't following the pre-cat O2 so that isn't indicative of a bad cat. Just one has the "jerky" waveform, while the other is smooth. I'm actually wondering if I don't have some other issue like bad spark plugs or a hanging injector, but since the waveforms on the O2s are different I figured I would start there.

AmDWs6

i wouldn't bother too much with what you are facing but if i were you and have the budget then yep..i'd do headers/ory.

Jonota

I'm not someone who likes to throw parts at a problem. I like to understand what the issue is and then troubleshoot to actually fix it. I'm uncertain that my issue has anything to do with an air leak: which is the only thing that the LTs and ORY would potentially fix. I'm not getting much response on the O2 cycles though. Another thought I'm having is the emissions equipment hooked up to the manifolds might be introducing some air. I've had that happen in an LT1 car.

LilJayV10

iTrader: (39)

I am assuming you have access to a scanner since you said you were looking at the wave forms.

What are your STFT's, that will help tracking down the problem.

Jonota

STFTs are ALL OVER the place, which is why I was leaning air leak. Everything looks good though. I attached a waveform screen capture and the actual scan file, is it not showing up?

LilJayV10

iTrader: (39)

While you are watching the short terms spray carb cleaner around the intake ports and hoses and see if the short terms go negative. IF they do you will know where it's leaking at.

GMRL

Is that sayin .07 on b1s2?
A good converter will be at around .7 volt, similar to what b2s2 is doing.
It may be possible that the converter is going bad or an exhaust leak somwhere may be triggering the o2 sensor to show lean.

Jonota

Just to update anyone interested:

Did a paper test at the manifold to cat union, it was severely leaking. Replaced with long tubes and Y-pipe (pacesetter). Install was smooth, did basic tune out of associated codes, removed AIR system. Did a seafoam treatment through the PCV port. After several tanks it is up to ~23mpg.

Today I decided to check the torque on the intake manifold bolts per another LS1tech poster. Found all but one VERY loose, most took more than 2 full turns to get to 89in-lbs. I could visibly see the manifold suck down.... oy. Hopefully that will give me the rest of the mileage I'm missing.