The replies I made in that thread (I didn't reread all the replies) only apply to the A/C system.
The A/C system has the ability to give fault codes only for its needs (some of them are engine related). It also has the ability to give many "actual values" which do apply to the motor such as engine speed, engine temp, vehicle speed, etc. While these can be interesting and even usefull they will not give codes on engine management, HFM in this case. It also will not give codes on the DM (diagnostic module which turns on the light) or the EA/CC/ISC (electronic accelerator/cruise control/idle speed control - this car might have it in the HFM controller) which can cause the light.
These modules were sorted out when the system was changed to ME in about 1997. The ME consolidated all fuctions into one module...one set of codes...FOR ENGINE. The new models have many systems with controllers that have self diagnostics. There are these basic groups: Engine, Body, Chassis, Climate control. Some cars have 10 or more controllers in the body section.
Checking codes in OBDII cars is available rather cheap (with various scanners/software) because of the government mandated standardization. This does not apply to the 10 to 30 or more controllers running the rest of the car.
Bosch Master, ASE Master, L1
33 years MB technician