Exhaust leak pre cat..... Where to start

[plyhammer]

Ok, Have an 09 535i It has an obviously loud exhaust leak and is running the code for lambda pre cat 02 sensor. Am I right that the leak would cause it to throw this code as the computer will think it is running lean and increase the fuel? It has a lot of black "soot" coming from the left side of the two exhaust tail pipes. I believe I will be hooking up the shop vac outlet to that tailpipe and try finding the leaks with the soap method.
Am I on the right path and what should I narrow it down to? Has to be between the engine and the upward 02 sensor so what do I all look at? Any helpful hints would be appreciated. Attack from the top or bottom? Thanks again.

[pbonsalb]

Left side is always sootier due to the vacuum valve closing the right side most of the time, if I recall the normally closed side correctly.

The leak near the engine bay is what I would focus on. Could be your downpipe to midpipe bolts have rusted out. A pain to fix. I did it as part of major maintenance - replacing turbos, hoses, oil pan gasket, rod bearings. I dropped the entire exhaust and cut/drilled out those bolts and replaced with regular bolts and new gasket. Heavy exhaust!

02 problem could be bad 02, could be intake leak, injector problems, misfires, etc. Not necessarily an exhaust leak.

These are great cars, but most need a lot of expensive and/or time consuming maintenance at this point. Have you changed all your fluids? Walnut blasted the intake ports? Fixed the oil leak(s) on the front of the engine?