Thoughts: Left bank misfires + bad cat and o2
 

I have been fighting some power issues the last few events and finally brought it to a trusted track friendly dealer. The car had a CEL, about a left side pre-cat o2 sensor. Mechanic says the CEL for the o2 was masking the real reason for the CEL, a left side cat had gone bad. In addition, the car showed 10x the amount of misfires on the left bank when compared to the right bank. For example, the car will record around 50-100 on average misfires over a given period, my left bank was averaging 1500.

When cold, no misfires were presenting themselves, which makes it near impossible for the mechanic to replicate the issue and log in real time the misfires. The car simply can not be driven hard enough.

The thought process is to replace the cat and potentially the o2 sensor, take it to an event and gauge any perceived power issues and monitor for a new CEL. If the issues are still present, or the CEL comes back on, re-log the car and check for a history of left bank misfires. Evaluate and diagnose from there.


I would think a misfire caused the cat/o2 issue, not the other way around. But I suppose it's easier to fix the known issues first.

What are you guys' thoughts on this? Anyone come across a similar issue?

My early guess would be fuel related. A bad injector with low fuel could cause misfires and certain cylinders to run lean. Lean burn is hot and then could cause a cat failure.



However, O2 sensors control your air fuel ratio and could also cause it to run lean. If I were you I would replace the O2 sensor and check your injectors first.



I would think a post cat O2 failure would indicate a cat failure rather than a pre-cat. I do not think that replacing the cat would do anything for the misfire issue.

What is the code number..

Onstar can also diagnose and give code number

I agree with your mechanic's logic. Honestly, what else could you do? If it was a bad coil pack, you would have random misfires and it wouldn't be related to the entire bank. I suppose the ECM or connection could be the issue. Replacing the cat and the O2 sensor on that side is the right thing to do at this point and see if this remedies your problem. Do you have a code besides an 02 code?

I had at one point, but don't remember what I pulled, and I didn't ask for the specific code from the mechanic.

I was going rich in one bank, pulled plugs and found one was black (rich). Wound up being a bad coil pack on that cylinder. I have misfires turned off though due to cam.

Crazy thing is it would be ok on dyno but instantly go to crap on the road tests.

Unburned fuel would explain torched cats. Could also be difficult to reproduce when cold. Had a similar issue on my F150. Port injector was sticking, sending unburnt fuel to exhaust. It was igniting when it hit hot cat and destroyed it. My codes were single bank misfires and faulty cat. A bad coil could absolutely be root cause, and could be aggravated when engine bay (and coil) temps rise, increasing electrical resistance.

I will bring up these points when the new cat arrives and see what he wants to do. My guess, because it is under warranty, they will do one thing at a time and reevalute.

Has any more info come about

My zo6 has been doing the same as of late

Left bank running rich o2 code

Im still waiting on parts

Maybe try swapping coils from bank to bank and see if the problem goes to the right bank.
Or suggest it to the mechanic looking at it

Great idea.. thanks

Just an update, my left cat was completely burned through. Had a nice hole you could see right through. Putting a few hundred miles on the car and then well reassess.

Thanks for the update

My cel went off .. not sure as to why .. could be a wire issue or else for a number of reasons I'm sure..

Do you guys battery reading fluctuate from 15 down to say the upper 12s?