Altering the size of the pipe will throw the calibration out. The air speed past the sensor cools the wire, which affects the resistance and the computer can work out from this how much air is entering the engine. If the pipe is bigger, the air for a given volume will move slower, so the computer will think less air is going in than is the case. At best this will cause it to run lean, and if the calculated airflow is too far from what it is expecting for a given throttle opening & rev range, then it will recognise this and set an error code.
And yes, the flat pieces are to reduce turbulence for more accurate readings, and the gauze is to keep out the house bricks.
Hope this makes sense, and helps.
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