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So, I had my ma's car this morning (since mine runs like crap and uses a 1/4 tank every 40 miles) and I thought I would try to get an OBDII on it. I stopped at my local oreilly's and got these codes:

P0741
Torque converter clutch solenoid circuit performance or stuck open

P0420
Catalyst system efficiency below threshold (bank 1)

What to replace, where to start? Any advice is greatly appreciated. I think the P0420 explains why exhaust comes out thick from only the left tailpipe and smells rich (or lean, don't really know the difference its just strong)

741 is when the PCM commands TCC solenoid "on" and it still detects a high level of torque converter slip. IT is a Type B DTC so it won't affect you passing/failing an emissions test.

The first thing to check, before going anywhere else, is to check the fluid level. Everything after this involves checking the TCC control valve for sediment, TCC regulator valve for sediment, TCC feed valve for sediment, oil pressure screen is clogged, TCC Pulse Width Modulator solenoid (may be called a PWM solenoid), O-ring is leaking. REally bad possibilities are that the turbine shaft seals are leaking or a torque converter is rupturing/ballooning. Eitherway, these are all internal components. Not fun!

420 is a fairly common code. But before diagnosing 420, you need to make sure that all other codes are cleared and not re-occuring. Basically, it is a system test to see how much oxygen capacity the converter can store. This is determined by the time it takes both the pre-cat and post-cat O2 sensors to "switch" to maintain a stoich balance (.475mV).

The pre-cat O2 sensor will "switch" quickly, bouncing back and forth between .900mV, to .200mV, back to .800mV, then back to .200mV, etc, etc, very quickly. Monitoring it on a scan tool, this "switch" rate will be almost instantaneous. Conversely, the post-cat O2 sensor should "switch" very slowly (or much slower) than the pre-cat O2 sensor. The longer it takes to switch, the higher the oxygen capacity of the catalytic converter (I.E. it's still good). If that post-cat O2 sensor is "switching" at a rapid rate, even as much as the pre-cat O2 sensor, that's indicative that the converter has a low level of oxygen storage capacity.

This doesn't necessarily mean the converter is bad though. Very likely, BUT, first check for exhaust system leaks before that post-cat O2 sensor. Check that O2 sensors are ok (I.E. no wires burned, installed correctly, etc). It is a possibility that the O2 sensor may be bad, but generally that throws it's own set of codes in addition, so if you don't have a slow response O2 code, or other O2 related, code, I'll presume the O2 sensors are ok. Lastly, check the Converter for physical damage, discoloration (such as high temps that will "blue" the metal, etc. If all checks out, THEN replace the converter.

My '01 Seville had the same TCC issue, but I never got it figured out or fixed (it also had a buttload of electrical codes it was throwing and the interior theater lighting and whatnot was out). *sigh* Northstar-system cars