"Lean" promotes misfire.
Misfire stinks. All the hydrocarbon in that cylinder is going straight out the tailpipe. In the process, it murders the catalytic converter, so the catalyst isn't removing the hydrocarbon traces that would normally be in the exhaust. Even more stink.
A crack in the driver's side exhaust manifold could "trick" the O2 sensor into thinking the engine was running lean, so that the computer goes to rich-command. Again, the excess hydrocarbon in the exhaust murders the catalyst.
And, yes, the O2 sensor is not to be trusted unless you connect a scan tool and PROVE that it's responding appropriately under all conditions. (It's easier and less hassle to just replace the silly thing.)
