You are correct it isn't a filter. What is does is measure the restriction in air flow the filter causes as the filter gets dirty. Rather then guessing how dirty the filter is or just changing the filter based on mileage or how it looks the indicator tells you the condition of your filter. With the cost of some of the filters today it can save a lot of $. Usually the truckers don't add stuff like this without a good reason.
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Cheers, Dan
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