letter K Leo Reynolds Flickr K Letter PNG Transparent Images PNG All K Letter PNG Transparent Images PNG All K&N Filter Minder Screw In Letter K Floral Initial Free Stock Photo Public Domain Pictures letter K Leo Reynolds Flickr. A Filter Minder works as air filter service indicator that monitor air filter restriction and indicates when the air filter should be serviced. As the air filter gets dirty, the yellow position indicator moves in the clear window of the service indicator and locks at the highest point. It can be read even after the engine is turned off. The air filter should not be cleaned until the yellow position indicator reaches the red zone.