Smart ForFour service indicator reset
This video shows you how to reset the service indicator / oil change warning on a first-generation Smart ForFour (2004-2006). It is the same procedure for all models.
If it doesn't work for you, note that the timing is important! Here are some tips:
You need to switch the ignition back on while the service indicator is still showing. If you're too slow here, it won't work
You need to hold button A down for ~10 seconds. If you hold it too long (or too short), it won't work. Note there's no visual indication for when you need to release the button; the service indicator resets AFTER the button is released.