OK, here's my take.

You have a bad O2 sensor. Actually a shorted O2 sensor (OR WIRE). I state all this because of your stated conditions.

1 - You are at the maximum lean correction. The lambda control basically is from about -12ma to +12ma. I assume you to be at the +stop on your system. A plus correction means MORE fuel and is a correction for a lean mixture OR low O2 sensor voltage (short to ground is as low as it goes)

2 - I base that on the fact that you read 0.0ma with lambda (O2sensor) disconnected. If other erroneous inputs had altered fuel correction, then the value wouldn't have been zero.

3 - you made mixture changes from good to bad in both directions (rich and lean)without controller output change (always at full lean correction. This part is conclusive and based on stated observation.

This of course is a normal car and you have made correct and full observations. You will of course understand that once one has proven a case the law of automotive entropy suggests other possibilities.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician