but I believe the A4 uses a film sensor, at least that's what it looked like when I put the air filter in. Anyone know for sure? Anyway, what the sensor does is measure airflow. It is a temperture regulated heater. When more air is flowing over it it takes more power is maintain the temperature, so by measuring the required power you measure the airflow. The resistance of the sensor is also proportional to temperature, so not only is it the heating element, it is also the measuring element.