Last week I got an SES light on the way home from work. I took it to Autozone and the code was: P0420 - Catalyst System Low Efficiency. I took it to a friend of mine who has access to a diagnostic tool and he told me the driver's side catalytic converter is bad. He also reset the code so the SES light would go out.

Since the cats are covered under warrany until 80k miles (I have 45k on the car) I decided to take it to the dealer to get it replaced. The problem is the service guy said that I have to wait until the light comes back on before they can fix it. That along with the trouble code data is also stored related to the problem and was also cleared.

This is the second time the SES light has come on. The first time I just reset the pcm by pulling the fuse before getting it checked so I don't know if it was the same code.

And last time I had it on the dyno she ran super lean, didn't drop below 17:1 the whole run with the sensor reading from the tailpipe.

 

I think you'll just have to drive it until it registers again.