VW Passat mass air flow meter. A mass air flow meter measures the amount of air that the engine takes in so that the correct amount of fuel can be added. They can be physically damaged if unfiltered air is allowed into the intake, or contaminated if oils are drawn in with the air. If yours isn't reading correctly it will cause your mileage to drop or the engine to run poorly, replace it.
Protect your sensitive VW Passat mass air flow meter with a working, clean air filter. And if you want to keep saving money purchase a VAG-COM scan tool. It will get the fault codes from your engine computer and save you a trip to the dealer if your mass air meter begins to act up again.