On-Board Code Reader- 94E420- codes & reset

Don't know where you live, but in the US, all cars of that era and forward have a universal system which has a std connector and can be read out with any of the aftermarket code readers. One company is Actron. These readers will read and reset the general codes common to all cars, ie cylinder X missfire, air/fuel mixture too rich, fuel evap system leak, , etc. There is manufacturer/model specific additional info that takes a more specialized reader. An example would be some cars have climate control system info that a more specific reader can access. But for DIY the Actron type works very well

I my 94 E420 has a code reader in engine compartment (just behind coolant reservoir gen area)

To find out what a particular code means, you can check the websites of the code reader manufacturers, like Actron. I've also gotten a lot of info by just typing in the car/code with a google search of both the web and newsgroups. Search would be for "E420 code PXXXX", etc.

There is online source... the basic gist of it is by your battery in that rectangle diagnostic port... there is a button. With ignition on, I think you press the button for 2 seconds and it will start blinking out codes.

Do a seach online... W124 obd.