I have looked into the ways that various types of tire pressure monitoring systems (TPMS) work. It seems that the typical factory installed active monitoring systems use pressure sensors that are mounted on the inside of the tire rims off the end of the valve stem assembly.

After market systems that I've seen so far have screw on caps for the valve stems that are the sensors. I deduce that for these to work they have to depress the post on the one way valve (a.k.a. Schrader Valve) to expose the cap sensor to the air pressure in the tire.

Are these after market type systems considered safe and are they legal in places such as the US? Since such system is so much easier to deploy than installing special valve stems with sensors inside the tire it would certainly be attractive to the car manufacturers, so are they driven by legal issues?

  • My purely speculative guess would be that this would not pass most state inspections. My reasoning is most state regulations are written to the standard at the time the rule is passed. This means that most regulations state the TPMS light must be off. The light being lit, and an aftermarket system installed still leaves the light lit and a failure.
    – mikes
    Dec 27, 2022 at 22:46
  • @mikes - If the vehicle in question didn't come with a TPMS to begin with, there'd be no dash light. I think that's what the OP is getting at here, adding a TPMS system to a vehicle without one. Dec 27, 2022 at 23:59
  • I agree if the system was not available when the vehicle manufactured. The OP questioned their legality in the US.
    – mikes
    Dec 28, 2022 at 2:11
  • I actually am wondering about this in relation to a old 1998 vehicle that has the passive type of tire monitoring system that always shows the tire fault light on the dash. It seems that the wheel speed sensor on one or more wheels has failed and it could be more expensive to repair than to add on one of these after market products. I suppose it would be another question here to query about replacing wheel speed sensors. Dec 28, 2022 at 4:36
  • A 1998 what? You really should ask the other question. Wheel sensors really shouldn't be too expensive. If one of them is out, I'd venture to assume the ABS isn't working either (or has a dash light on for it)? Dec 28, 2022 at 23:35


You must log in to answer this question.

Browse other questions tagged .