Just how critical is the check engine light? Mine came on today while I was on the highway. Called the dealer and they said since there are virtally thousand of sensors within a MB,it may means nothing. I insisted to have the car checked, and they told me the computer read out says its a default O2 sensor. Are they pulling my leg? And what does O2 sensor for the car? My car is a 95 SL320. After the dealer rigged up all those cable to my car, the light went off, They told me they will replace the O2 sensor anyhow since its under warranty. Any help on this is appreciated so I can sleep better knowing the car is OK.

The check engine lamp is a "Government" requirement for the emission control system. The oxygen sensor (O2) tells the fuel management system how much fuel to sent to the engine. Sounds like your dealer performed a good diagnosis.