[Britwrench]

Firstly, you don't just "lean it out" Motronic systems rarely go out of adjustment so you diagnose and then repair. Find the fault fix it, that easy!! Disconnect the 02 sensor if you want to check it...the engine will run slightly rich, but not rich enough to cake the plugs.

Bearing in mind the number of times we've seen dead 02 sensors, and that haven't worked for more than a while, I wouldn't class it as the #1 cause of problems..only to pass emission tests do you (usually) need to work.
If the engine has a vacuum leak the voltage reading will be wrong too..lots of possibilities...