My check engine light just came on without warning. Car has been recently
serviced. What could be going on?

Could be any multitude of things. First step is check the flash codes or go
by some place like Autozone and they will read the OBD for you. If the car
is running OK then it might just be a marginal sensor. To read the flash
codes, go to the OBDI interface near the battery and hold the button for 2 -
3 seconds to start to read codes. Reading codes involves just counting the
times the LED blinks, and then correlating that with published material
(searchable on the web or let us know the codes). Then hit the button for
2 - 3 seconds to get the next code, and continue until the first one
repeats. When you want to clear them, hold the push-button for more than
six seconds.

Josh

I have a 1994 E320. My "check engine" light comes on about
once or twice a month.

Josh gave you good advice on reading out the cause of the light.

After doing this quite a few times, I just routinely:
(1) check the oil dipstick to see if I'm low,
(2) reseat the gas cap,
(3) reset the "check engine" light to off,
then start up the car. The light has always stayed off a while.

So far the gas cap has been the problem every time. Always be
sure to really snug it down every time the tank is filled - even
a fraction of a millimeter short will light up the check engine
error light on my car.

Elliott Moore