My 94 960 with 140k miles developed an exhaust leak noise. Inspection revealed that the downpipe was cracked about two inches forward of the cat converter. It had been welded shut prior to my purchasing the car about four years ago, and the crack somehow reappeared.

At about the same time the car experienced a number of other problems, including a check engine light with a 212 code (O2 sensor), engine skipping under load, and periodic black smoke. A mechanic removed and rewelded the crack in the downpipe to seal the exhaust leak. The upshot is that his repair corrected all of the aforementioned problems.

I can understand how ambient air leaking into the exhaust system can introduce an oxygen rich mixture in the exhaust gases and confust the O2 sensor into thinking the fuel mixture is too lean, causing it to compensate by enriching the fuel mixture and causing the symptoms. My question, though, is how a crack in the exhaust pipe that's downstream of the O2 sensor can let outside air flow far enough back up the pipe to affect the O2 reading? It seems that there would be so much exhaust gas pressure when the engine is revving under load that there would be no way for outside air to work its way back up the pipe against the flow. Does anybody know how this happens, and what are the implications if I develop another exhaust leak? Would an exhaust leak behind the cat converter potentially confuse the O2 sensor, also?

Robert