you need to try to get someone to walk you through reading the "code" to verify but I would bet it is the O2 sensor. I just replaced the one in my '95 E320 with around 85,000 miles. Same thing, light was on one morning and car ran great, got good gas milage and the light stayed on anytime the car was in use until I replaced the O2 sensor.
I do not know if the E430 has the built in code reader but I will explain it on the E320. Right in front of the battery is a little rectangular black box with a bunch of holes in it (It may have a cover over it, lift it off if it does). Looks like you could put a Fluke meter probe into them.
At one end of the box is a red LED light and a push button. Put the ignition in the "on" position without starting the car. Hold the push button down. When you first push it down the red LED will turn on then go off. Keep holding the button down. If there is a code (there will be) the light will start flashing. On the E320 12 means that the O2 heater circuit has gone bad and you need to replace the O2 sensor.
If you need help on replacing just let me know.