I removed exhaust manifold shields, started car cold and had a helper stuff a rag in the tailpipe. I could not feel or see any exhaust escaping around cat, flex pipe, turbo flanges or from or around exhaust manifold. I did not see any tell tale black streaks on the manifold to indicate a crack. I forgot to mention that vacuum at the intake is a steady 17 in hg (which is probably normal for 139K?)& at idle, fuel pressure is a steady 37 psi at fuel rail. If an exhaust leak is not fooling the 02 sensors, what is causing P0172 (system rich)? Klaus, could a fuel pressure regulator and/or damper cause this? Still stumped, Phil