The Mercedes E320 is equipped with a computer system that reads trouble codes from sensors positioned throughout the vehicle. When the "Check Engine" light illuminates on your instrument panel, you need to have the engine looked at right away by a qualified mechanic. You should have any necessary repairs done before you attempt to reset the light. The mechanic should do it for you, but this isn't always the case. And if you do the repairs yourself, you need to know how to manually reset and erase the code. It's an easy fix on a Mercedes, requiring no special tools.
Put the key into the ignition and turn it to the "On" position, but do not start the engine. Take note of the display on the instrument panel between the tachometer and the speedometer.
Located the four buttons on the left side of the steering wheel. Select the lowest of the buttons on the left and press it. Notice what the display says. It should tell you why the Check Engine light is on.
Look for a button on the left part of the instrument panel. This is the reset button. Press and hold it until the display shows a message saying to hold the button for three seconds to reset it.
Release the reset button, then press and hold it again. Wait for the display panel to tell you that the Check Engine light has been reset.
You could always disconnect the positive battery cable from the battery for 30 seconds to reset the light, but this will reset the entire computer system on the vehicle. You would have to reset your radio presets and clock, too.