The BMW 3 Series has a built-in service engine reminder to notify you when you need to take your car in for routine servicing. When you turn the key in the ignition on, green LEDs will illuminate. Once you start the engine, if the green lights turn to yellow or red, then it's time for service. You can reset it yourself at home with a reset tool that can be purchased from your local auto parts retailer.
Put the key in the ignition and turn it on, but don't start the car.
Shut off the radio, lights and air conditioner fan.
Lift up the bonnet of the car and plug the reset tool into the diagnostic connector. This is located on the left side of the engine toward the front, if you are standing in front of the car looking at the back of the car.
Push and hold the "Service Engine" button on the reset tool. In about 10 seconds a light will flash then go out. When it goes out, let go of the button.
Look on the instrument panel to verify that the service engine light has gone off.
Make sure you take your car in for servicing before you reset the button to ensure that nothing is wrong.