I have read the many threads about the PO420 and the P0171 codes and the responses about this being potentially one of the following: leaking exhaust flanges, bad cat. converter gasket, bad throttle body, defective cat. converter, Iridium vs Platinum plugs, exhaust leak, poor fuel blend usage, and etc.

It would seem that my problem, like many others, is tied to potentially one or more of these issues and is causing my cat. converter threshold readings to be insufficient potentially.

Right now my Check Engine and Trac Off lights are lit up and Autozone suggests it may be P0420 and/or P0171

How do you reset codes on 1999 ES 300 to begin to see if this is something simple?

If the lights come back on what is the best course of repair?

 

I moved your post from the RX300 forum to the ES300/330 forum.

What codes actually showed up, P0420 and P0171?

You can reset the lights by pulling the negative battery cable for a few seconds. AutoZone should be able to reset them as well. The final trick is going to the fusebox in your engine bay and locating the EFI fuse and pulling it out (with keys out of the ignition, but if you forget, don't panic) for a few seconds, try 10, and then putting it back in and that should reset it, but if something is truly wrong then it will come back. Some of these things are just temporary sensor readings causing the issue.

P0420 is usually a bad O2 sensor, bad cat or a temporary condition. P0171 is a lean error condition, but usually found in conjunction with another codes (usually an A/F sensor error, you rule the P0171 out and focus on the other code).