The knock sensor is a very good sensor to have. It is basicly a microphone for the ecm to listen for detonation, or knock, and depending on the severity of it, retard the ignition timing accordingly to prevent engine damage. P28 and P72 have the same connections. The p72 is desirable because it is fairly easy to get, has vtec, and is easily chipped to run the b series vtec motors. You can get a p28 cheaper than a p72 in most cases.