You need to get your hands on an OBD reader. (OBD = On Board Diagnostics)
There is an OBD plug under your dashboard. You plug in the reader, turn on the ignition, and the error codes are downloaded to the reader.
Your local auto parts store may have a loaner you can use.
Out of curiosity, what year is your MO and how many miles are on it?
2005 SL AWD, Platinum
-OEM HID conversion & HID fogs
-OEM iPod interface
-Upgraded (non-Bose) speakers - Pioneer TS-A1702C & TS-G1643R
-Muth signal mirrors
-Valley Industries hitch and wiring harness
-Aluminum fuel tank shields