In the last test I mentioned, the concept is to run the car and evaluate the mixture by seeing how it affects the O2 sensor voltage. The sensor is a 0 to 1v voltage generator. It creats voltage that is related to engine mixture within a small range.
Its very likely that the sensor is fouled, but the way to tell is to run the motor (the sensor has to get hot which requires time and engine speed - don't be in a hurry), while monitoring the voltage (sensor on its own not connected). As I stated its likely to be either 1v or zero volts. If its one volt and its working then the mixture is greater than 1% CO (if measured before cat with an exhaust gas analyser - which you really can't do anyway). Here is why I suggested to do the 12v thru Cap'n mixture change. This will lean the car out and unless your are very rich it will bring the mixture through the range 0-0v - 1.0v. if not you can lean it out till it does.
Remember a proper working O2 sensor measures mixture from 0% CO to 1% CO. This is a very small range. The car will start running poorly above 6% CO and below 0.2% CO (maybe higher). The car will be held to mixtures in the range of .3% CO to .7% CO by a functioning system and it can only correct a rich car running less than 4% CO.
If everything is functioning you will be able to lean out the mixture till the O2 sensor just drops from around 1v. Then you will be able to reconnect the wire and the system will correct mixtures less than .5v by adding fuel (within the 10ma correction range we have already tested) till the mixture becomes richer than .5v at which time the current will reverse and the system lean out till it switches the other way. Once the O2 sensor is hot and everything is adjusted then the .3v to .7v O2 sensor swings will be kept in check by about a 4ma total mixture correction.
If this were a training course this would really be a short story. If you don't understand ask me again.
Bosch Master, ASE Master, L1
33 years MB technician