Diagnostics
All W124 models are pre-OBD2, which makes extracting diagnostic trouble codes difficult if you do not have the right tool. In summary, there is multiple computer modules which can contain codes. Each one must be checked individually. The diagnostic module is related to the Check Engine Light(CEL) which contains emissions related codes. You can read and clear codes from the Diagnostic Module using the pulse counter located near the front passenger strut tower.
To Read Codes
At the impulse counter, hold and press the red button for 2-3 seconds. Once released, count how many times the LED flashes. The number of flashes corelates to the trouble code. There can be multiple stored codes so repeat the process until a code repeats. A repeated code means you have cycled through all of them.
To Delete Codes
Repeat the process to read a code. After counting the flashes for the code you want to delete, press and hold the same button for 8-10 seconds. You can only delete one code at a time.
The link below redirects to an external forum site where a member explains in detail the process of checking DTC's.
https://www.500eboard.coMore resources coming soon.