Take it to AutoZone for a quick, free diagnosis. they may say they can't read a Mercedes but you need to invoke the secret word "O B D 2" and they will be surprised that they CAN do it.
RE your questions, 1.) I dunno; 2.) the car stores the fault codes so it should not matter if the light is on or not.
RIP "Betsy" (Oo=*=oO)
'96 E 3 2 0 (W 2 1 0) M 1 0 4 (Totalled)
Spaces in sig so as not to screw up the SEARCH; every time someone searches for that MB they don't want my sig!
2004 Audi A8L
'98 VW Passat 1.8T 5M
'87 Alfa Romeo Milano 2.5L 5M
'67 Impala convertible, 327cid