I have been chasing O2 sensor issues for a couple weeks now. I could use a little help.

Vehicle: 2001 Pontiac Firebird Trans Am WS6 M6. 43,000 original miles

I recently bought this car from a dealership in Ohio and had it shipped to upstate NY. They welded in cheap cats in order to sell the car. They didn't hook up ANY O2 sensors.

Setup: BBK LTH. SLP or BBK y-pipe with cheap cats welded in (originally traded at the selling dealership with no cats). It has an SLP loudmouth exhaust of some sort. Not sure if it's a I or a II.

Initially, I had MANY O2 codes. I had a local guy tune out my rear O2 w/ HP Tuners. After that, I only had P0134, P0154 and P0300. I replaced the spark plugs and wires. I replaced the very old original & working upstream O2 sensors w/ extension harnesses as preventative maintenance. I used brand new upstream AC Delco sensors for this application. I probably should have gone with C5 Z06 sensors w/ longer wires so I could delete the harness extensions. P0134, P0154 and P0300 have not come back! Thank God. It ran well enough, for long enough, to pass the NYS inspection.

After about 50 miles I had P0153 code on Bank 1 Sensor 1. I had the dealership I work for move the sensor (brand new AC DELCO) to the other side in order to isolate any possibility of wiring issues. The code came back on Bank 2 sensor 1. This told us that the brande new sensor was faulty. We replaced this brand new sensor under it's parts warranty with another brand new AC DELCO sensor.

A week later P1153 pops up. 02 Sensor heater circuit high voltage bank 2 sensor 1. I've never had this code pop up. It's almost always a insufficient activity sensor code or insufficient voltage.

The car doesn't run as well as it did a week ago. The idle is rough and it's down on power a little. These symptoms persist even after I reset or clear the codes. There is both an exhaust smell and raw fuel smell. I've had my dealership check the exhaust for leaks and they can't find anything obvious with human eyeballs. We didn't try soapy water or anything like that though.

I'm not sure what to do next. This thing has had three brand new sensors. I don't think the sensor is the issue. I'm thinking maybe the tune is WAY to rich or maybe there is a manifold/header leak by the heads? I don't think any leaks past the upstream O2 sensors would cause a code.

I do not like modified vehicles for this reason. But damn it...this thing sounds so freaking amazing...I don't think I can live without the long tube headers now lol!

 

I’d try looking for vacuum leaks. Carefully look at all vacuum lines, their connectors, what they connect to. Also inspect your headers for holes (it looks like there is some corrosion and make a hole in the bottom of the collector in your photo). Unmetered fresh air can make your car think that it’s running lean and the car will start adding fuel to compensate and start running rich.

 

I also have a P1153 code after American Racing longtube headers and cam. Car was dyno tuned by Justune here in KC area. I have replaced the oxygen sensor and the extension cable and it still pops up after I clear the code.
Car exhaust smells rich and car runs like crap with the code pops up. I don't want to just program the code from not showing the yellow "Service Engine Soon" light. I want this fixed! Has anyone had this happen to them?