I have recently had a mechanic install an aftermarket cat. converter on my 2001 SSEI. The car will still not pass emissions in Massachusetts. He has tried fooling the system by using different capacitors wired to the O2 sensors with no luck. Obviously I am trying to avoid the VERY expensive factory Cat system. Any Ideas or help about these capacitors to fool the emissions machine (and the "check engine light?)

Do you know what brand and model number the cat was?

Lots of people run after-market cats, without issues. There should be no reason you have to get an OEM cat. Your mechanic should be spending more time finding out the problem with your car, instead of trying to fool the machine, which he is not likely to do.

Your CEL light is on? What is the code? Have all of the regular maintenance items been done? How is the O2 sensor, plugs, wires? Throttle body been cleaned?

Your mechanic friend isn't schooled up on electronics all too well. O2 sensors generate an AC signal which vary in millivolts. Capacitors store positive charge and release it when the incoming charge becomes weak. Meaning that AC signal is resembles something like a sawtooth waveform....something the computer will have absolutely no use for.

Now then...is your new cat smaller than the factory? The exhaust might not being scrubbed enough when passing through your new cat, and your rear O2 sensor is noticing that and throwing the code. MAYBE.

Thank you for all the help. I had no idea people liked the Bonneville as much as I do! The CAT is a 49 state unit, whereas the original is 50 state. The cost difference was huge but now seemingly foolish. The idea behind the capacitor was to show a greater difference in millivolts from the front O2 sensor to the back O2, thus showing the computer that the cat is functioning. Apparently that is not logical thinking. Both O2 sensors are new, and a tune-up completed recently. The code still comes up as the rear O2 not functioning.

Any recommendations on an aftermarket cat that I can install, pass emissions, and not hinder performance? Thanks again.

If this is the only code that comes up...your cat is probably working fine. If you have a warranty on that rear sensor (I presume you bought that new), I'd replace it and clear all codes. A word to the wise...Bosch O2 sensors are, for some strange reason, very short lived in our cars. Get Delco or Denso.

OK, so he was connecting the cap between the sensors? The interaction between the two signals would make it unlikely that would work. Sometimes it would be additive, and others subtractive, and would still throw the code. Like Sandrock said....AC/Delco or Denso only. Bosch, Borg/Warner and others just don't hold up.

Yikes...I hope your mech didn't fry a wire, the PCM, or something else (maybe the rear O2 sensor itself).

I also wonder if he plugged the rear O2 sensor back in after screwing around...did you visually inspect to make sure of that?

Good point. Where was he making these connections? Did he cut into the O2 sensor wiring? That is actually part of the sensing circuit, so if he did - he likely damaged the sensor.

Please put the year make and model of your car in your sig so we can help you better

If all else fails you can get a computer reflash (depending on your year) which will disable the rear 02 DTC check.