P0420 After Cam Install

Tuning an LFX (3.6 V6 DOHC) but posting this here since it'll get more views.

Awhile back I installed Mace cams and within a few weeks got the P0420 code (hadn't done any tuning at this point and was still running a tune from my previous tuner). Figured it was due to running rich (was 10 - 15% rich across the VVE table when I first started tuning), but now that the fueling is within 2% everywhere, the factory O2 sensor is back in and I haven't been downloading a tune every day (engine light/codes reset), the code eventually came back. Believe the post cat sensors are indicating that the cats are functioning properly as they show a mostly constant voltage, but the voltage is a bit high. While driving at steady state, bank 1 is averaging about 765 - 770 mV while bank 2 is averaging 690 - 695 mV. I'm not getting a P0430 code (bank 2) so I wonder if the code triggers when the average voltage is over 700 or 750 mV. Unfortunately I didn't do much logging before the cams so not sure what the voltage "should" be. I also have high-flow cats from Solo but have been running these for a few years with no codes.

Noticed in one log that when the intake cams advanced (went from cruising to ~50% throttle) the post cat voltages dropped, so I added 4 degrees to both intake and exhaust in the cruising areas (extra 8 degrees overlap) and the voltages for both banks started averaging under 700 mV during the same cruising speed as before.

Anyone know if adding extra overlap is a solution to the P0420 code? Is the higher voltage indicating that I should be adding more overlap? Or is a higher voltage just a result of the high-flow cats and cams (and/or VVT tuning), and there isn't necessarily anything wrong with having a higher voltage? I'm guessing the voltages raised a bit compared to factory after installing the cats but as mentioned I didn't do any scanning. I know I can just turn off the MIL/set to No Error Reported, but not sure if this would still prevent me from remote starting (code hasn't come back since I reset it yesterday so I can't confirm yet, but guess I'll find out in a few days). In any case it would be nice to get rid of the code without just disabling it.

The P0420 code will not go away until you replace the cat. It only takes about 30 sec to destroy the cats on this engine.....usually due to worn timing chains and the cold start misfire code that will pop up due to carbon build up. You can just turn the code off and no problems. you can still remote start.

P0420 and 430 are very common with high flow cats. No high flow cats work as well as factory ones hence the codes. I'm surprised you ran them for years without issue.

Figured I'd update this, kinda...

By driving around for a bit with some overlap at cruising it looks like the P0420 code has disappeared - just had the light pop up so read the codes, and P0420 is no longer a permanent code. FYI was also was able to remote-start the car with the code conditions present, but the code disabled.

My original tuner (Overkill) advertised that his tune would/could get rid of P0420/430 codes, and I figured he wasn't just disabling the codes; guessing he did this through the VVT.

Mind to post tune and log?

Attachment 156755

I've got at least a couple good posts on here about catalytic converter function and monitoring if you want to search for them.

The downstream O2s should hold a steady voltage of about 7-800mv during cruise once they "light off". This is when the converter is storing O2. The sensor reads "rich" but it isn't. It's oxygen deficient. On decel, the downstream should drop to near zero about 1-1.5 seconds after the upstream does. This is when it's releasing oxygen.

It's pretty interesting you have aftermarket high flow cats that actually work. That is a real rarity.