The mass air flow sensor tends to fail causing poor fuel economy and Check Engine light illumination. The engine air filter should be replaced when the mass air flow sensor is replaced; a dirty filter can allow debris to damage the new sensor.Google+
Car Problem Reports
Mercedes-Benz E320 Replace air filter when replacing failed mass air flow sensor
Mercedes-Benz E320 Problem
Visitor, 2002 Mercedes-Benz E320, 163,000 mi
I had just replaced the MAF during the last oil change but the air filter looked fine so I left it alone. About 2000 miles later I have the MAF code again.