Steve Brotherton has an excellent article in the DIY section, but there's nothing specific to a 91 420SEL. http://batauto.com/MB.html
is the website I was thinking of. It doesn't have any V8 engine codes. It also doesn't describe how to check your car without tools.
I took my SE to the dealer when the check engine light came on. The advisor pressed some buttons on or near the ECU, read code number <whatever it was> and claimed it to be the air temperature sensor. Sure enough, I had started the car with the air temp sensor disconnected. It was a similarly simple matter to reset the ECU without disconnecting the battery. No charge.