I had a problem with the old "red/green" indicator caps a few years back - the ones that changed color if the pressure dropped. I have trouble trusting anything that requires the valve in the stem be "open" as appears to be necessary the way the TPMS indicators work. If the cap leaks... you lose your tire pressure. The cap gives a little bit of a backup to a slow leaking stem valve. The OEM versions (in cars anyway) measures pressure on the inside of the tire, the stem valve remains closed, and a good cap helps to minimize leakage.