You need to make an LED code reader to scan the codes from the SRS pin (#5 for the 8 pin module on the w126 and w201) once you've read the codes you can erase them and see what comes back.

Here is a basic schematic for the LED code reader:

+ (pin 8 or battery) ----- (+) LED (-) ------- SW ----- (-) (pin 1 or battery gnd)

From the - side of the LED that is connected to the momontary switch go to pin 5 (or any other pin for diagnostics)

To read the code, with ignition on (engine off) hold down the switch for 2-3 seconds. Release - count the number of blinks on the LED. Repeat until the same code is read.
To erase code, first read it, then hold down the switch for 8-10 seconds. Each code must be read then cleared individually (there is no clear all).

Sometimes to clear codes you have to turn the ignition off then on again between clearing separate codes.

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In the family: '03 E500 // '04 ML500 // 64 220SE