Filter Minder gauges takes away the guesswork when it comes to knowing if it's time to service your air filter. These gauges monitor airflow and the gradual increase of air filter restriction. When the air filter is clean and has little restriction, the needle will stay in the green section of the gauge. As the air filter starts to load with dirt the needle will move into the yellow section. Once the Filter Minder needle reaches the red zone it's time to clean your filter and reset the Filter Minder gauge.