Ok, I picked up my 03 MCS about a week ago and need to get it inspected but it threw 5 SES codes since I bought it (none the first 2-3 days). The codes are P0171 (twice), P0131 (twice) and P0136 which all relate to a lean situation. At first it was just the first O2 sensor code so I changed that, but no difference even after a computer reset. So here is my question: I did dump a bottle of fuel system cleaner in right before the light came on. What are the odds that the bottle of cleaner is what is causing the SES light to be on? I'm only about half way thru the tank of gas but I'm getting ready to drive it around for an hour or two and then put fresh fuel in it. Anyone else ever hear of an additive turning on an SES light on one of our cars? Some additives claim they can eliminate the code so this seems far fetched, but if it's not that then I'm thinking I have a bad air leak somewhere in the intake or exhaust and I haven't found any of those either.

 

Have the catalytic converter checked. I will say with a 95% surety that it is shot. Its most likely clogged at the point where the exhaust manifold/flex joint is pressed together with the catalytic converter.

 

Check for exaust leaks....a crack BEFORE the o2 sensor can let oxygen in, producing lean codes..... Easy to check....
When the exaust is given an OK, with no leaks, then you can look deeper if nothing was found.

 

So no codes have returned and all seems to be well. It passed inspection with flying colors. Now to have some real fun working on it. Sounds like it's time to service the SC and while I'm in there replace the water pump. Everything works great, but it does make some noise and at 111k miles I'd rather be safe than sorry. I guess I know my next weekend project. Thanks for the help guys. I'm guessing it was just a loose intake tube as I never found any other issues although I did clean the PVC valve and some other pieces so maybe it was everything together.