usually it's the pre cat sensors that the computer adjusts from, the post cat sensors are just there to check cat condition, theres two different types of sensor that should be used in that general year range of euro cars, one makes it's own voltage (easy to check with a graphing meter and propane torch, and my bet that's the type used on that car) and the other manipulates the voltage signal from the computer. with a graphing meter set to a 1 dc volt hit the sensor with the torch on a decent flame, with in about 30 seconds it should be over a half volt, after a minute or two of heating the sensor you can start playing with it removing the flame the voltage should drop off sharply and to near zero with in under a half second (my snapon vantage says 10th of a second) on the graph it should go near straight down, the voltage should be above .850 with the flame on it, if the reaction time of the sensor gets too slow that causes problems. the other type of sensor needs to be tested on the car, you can play with the throttle and watch the readings
all america wants is cold beer warm cat and a place to take a poop with a door on it