It is very hard for a professional to give the go ahead to drive with a "check engine" light on. So let me tell you what the dealer told my wife when her 18 mo. old ML320 had a light on: "You can bring it in Wednesday" (10 days from phone call).
They didn't consider it an emergency and I verified for myself what was wrong. It was an O2 performance code. One that occurs after an OBDII monitor test is performed. They replaced the sensor under warrantee and all is well.
Bosch Master, ASE Master, L1
33 years MB technician