Hello,

I have a 2001 Pontiac Trans Am WS6. I fabricated a new exhaust system for it, and I put in two new high flow catalytic converters, one for each side. About one hundred miles into driving it I got a code for inadequate catalytic converter efficiency on bank two. I looked at the real time data, and the post cat sensor was acting like the pre cat sensor (jumping all around). All over the internet folks say that means a bad catalytic converter. And it makes sense to me. The same thing is being read after as before because the cat isn't doing anything to the exhaust gasses. However of course I hesitate greatly to replace it since it is new. Additionally note that the other side got the same cat, and there was no code for it. And the real time data shows a steady post cat number. I even switched the two sensors from one side to the other, and I got the same reading--ruling out any sensor issue. Then I remembered that the old exhaust I took off had an aftermarket cat on bank two and an original factory on bank one. This means someone before me replaced bank two cat. Possibly they had the same issue. Also possibly they treated the issue by replacing the cat as I would under normal circumstances, and the engine killed that cat too (car had engine trouble codes and would not pass emissions before I got it and started on it, although I did not investigate them then unfortunately to have any details). So it is possible my engine is killing cats on bank two only. But I could be way off course in my analysis, really. I want to get anyone's opinion who thinks they might know what route I should go with this. I want to have a good plan before starting any work from here, although I do intend to take down bank two cat and visually inspect it internally for additional clues. Thank you in advance

 

Make sure one bank isn't running too rich. That will kill a cat in no time. It cooks them.

 

Sounds like a good thing to check on. I'll do that. Thank you

 

You could always get one of those cheap ebay O2 sensor extensions to fool the O2 sensor. I've used with good luck.

 

HF cats will never be the same as OEM cats. Turn off P420/430 codes and go on with life....

but I would check your trims also.

 

Thank you so much for these tips. I'll look into these things

 

I did not understand right away what you meant by the o2 sensor extender, but now that i know that does seem like the best bet now. I will try that, and once i have driven it long enough for the computer to throw or not throw a code i'll post the results. Thank you again for the input

 

I put in the extenders on both sides. It definitely helped to smooth out both sides, but the passenger side is still erratic, although not as much. Once i let it idle for a bit it will even out at about .6 volts. The driver side at idle is very steady at around .070 volts. I inspected the honeycomb of the cat on the "bad" side, and it still looks new to my eye. It isn't black or structurally damaged at all. It is a slight tan/yellowish sort of tint. It doesn't add up to me that both sides have the same new cat, but one side runs consistently at .070 volts post cat and the other consistently .6 volts post cat. Any thoughts would be appreciated.

 

If the car has headers has it been tuned?

If the wire extensions are long enough label them left rear and right rear. Then swap the left and right side connections for the rear 02's. If the low voltage swaps sides it could be that 02 sensor is starting to fail.

If that 02 sensor is bad ice read it's sometimes possible to clean them with brake cleaner. I've never tried it however.

 

Also my fuel trims at idle say -.8 for sensor 1 and 99.2 for sensor 2 for both sides fyi

 

I decided my next step will be to assume i have an injector malfunctioning and putting out too much fuel on that side and pull all the plugs on that side and check them to see which one is blacker than the rest. That would explain why i am getting a richer number, but still i could be wrong. I just don't have any other direction to go at the moment.

 

I'd try CRC electronic cleaner on O2's. Probably a bit more gentle than brake cleaner.

 

I'm not following what you are saying.

The -.8 makes sense but not the 99.2.

Normally what kills a cat is a misfire. It's actually the raw unburnt fuel from the misfire. That's why after a misfire gets so bad the CEL starts flashing. At that point the PCM has decided it is a catalyst damaging misfire.

A quick and dirty test to check the condition of your cats is look at the post 02 sensor. It shouldn't switch like the front. If the rear is switching and following the front. The cat isn't "holding" oxygen. Again an aftermarket cat is never going to be as efficient as an OEM.

Putting spark plug anti foulers works because the rear 02 sensor "sees" less of the exhaust gas which keeps it from switching rich/lean.

 

Thank you again for your input. The problem doesn't add up to me. It seems i am missing something. Let me recap everything and see if you agree. Bank 1 and 2 got the same HF cat at the same time. After one hundred miles of driving a code came up for inefficient bank 2 cat. I looked at the live numbers, and this was confirmed. Bank 1 is operating perfectly however by the numbers. The engine runs great, and both cats have turned the same color on the outside cover due to heat during that first hundred miles. There is no smoke out either side. There is no indication of bank 2 running differently from bank 1. I installed the o2 sensor extender on both sides post cat, and it did little to the numbers on either side. Bank one consistently reads .080 at idle, and bank 2 consistently reads .600 at idle, and even with the extender bank 2 will jump around a huge amount. Massive difference between the two sides, but they run the same way. I tried swapping the sensors from one side to the other. The problem stayed on bank two, which told me both sensors read the same way. I also criss-crossed the harnesses to rule out a harness problem. The computer read the same problem then on bank one. So i put them back, and ruled out a harness and sensor problem. I know for sure the two sides physically run the same way because my exhaust is a side-dump where each side is independent and dumps out behind the front tires. So i can compare them that way. I have no codes for misfire. I dyned it recently, and the power is on par (340hp 364 torque). I visually inspected the inside of the bank 2 cat, and it looks fine. There are no air leaks. I checked several times by taping the blower end of my shop vac to the pipe tip to create internal pressure, and i felt my hand around everywhere for air spewing out. On the first run i found a little spot, but i repaired it. No air leaks now and problem unchanged. That is about all i can think of. Thank you for your thoughts! Everything is the same on both sides, but they read like two different engines. That is what throws me

 

how far are the cats from the headers?

Something else you can check if you haven't is the temp on the inlet of the cat and the outlet. an infrared camera is really helpful to check that but a IR temp gun does the same thing.

 

It's hard to read without paragraphs.

Exactly what sort of exhaust set up does your car have? Long tube headers with cats? Mid length headers w/cats etc?

Can you post pictures of the set up?

 

The headers go to the triangular flange. The cat is one inch away from the flange.

 

Yes, i will post pics of the exhaust set up and also of my obdii readout

 

After you see my exhaust you will probably wonder if i have any ground clearance issues. I have not scratched at all yet. The end of my driveway is usually my catch all test for ground clearance whether it is a trailer or whatever. If it clears that it clears about anything, and it does