If you haven't already, check Steve Brotherton's article on engine control systems in the DIY link at the top of the page. You can check O2 sensor operation with a duty cycle meter through the diagnostic port. Reading the voltage directly off the O2 sensor is tricky because the O2 sensor reading oscillates in a small range between 0 and 1 volt in closed loop operation. Maybe your DVM has a mode that makes it easy to decipher such a reading.

Sixto
95 S420
87 300SDL