Per JimF's website - http://pages.prodigy.net/jforgione/MB_DM.html

2 = Heated oxygen sensor inoperative

20 = Speed signal not present

26 = Upshift delay 1 to 2 (model 123.034) , 2 to 3 (all models)

Has your engine wiring harness been replaced?

Sixto
95 S420
87 300SDL