There is no need or use for a gas analyser on that car with a functioning fuel control system and working cat. Nothing will be coming out the rear anyway in those conditions.
The system is designed to allow monitoring and control evaluation from the diagnostic connector. Proper mixture is easily achieved and closed loop function monitored there. IT IS the first place to go. if you are running in closed loop and have this problem things might get tricky, but it is my guess that you AREN'T running in closed loop....thus mixture adjustment is not the first problem.
If that doesn't make sense, look up closed loop control in the archives. It forms the basis for all modern engine control systems.
Bosch Master, ASE Master, L1
33 years MB technician