I am having an issue with TPMS, or ?

So, I had a completely flat tire this morning. Up to this point if any tire was low, the TPMS worked just fine. (Still not sure that is the problem) So, I got out my portable tire inflator, filled it up, 35psi. Tire size is P245/65 R18.

I took it to the local tire shop. They took it off the vehicle, checked the pressure, it was fine. Put it in the dunk tank, nothing. Sprayed it all over, the tread, the front, the back, the bead, the stem, nothing. No leaks. They did it over, and still, no leaks. So, just paid them for their time and left. Was just worried about it, took it to another tire shop, same thing, nothing. The Tire sensor light was off.

So, I found out how to reset the TPMS, did so, got all the beeps and chime when done. Filled up the tires to the proper inflation, 30 in the front and 35 in the rear. The Tire light inside went off. So, it seems the TPMS, is working, what else would cause the tire to go flat like this? I fear getting stuck out in the middle of nowhere. Could the pin inside the stem itself be bad, and just lets out air under certain conditions? If so, can those be changed out without replacing the entire TPMS stem unit?

Thanks for any help you can give.

 

the valve stem pin can be changed out. if you have a aluminum tps sensor...you need a nickel plated valve stem pin. if the valve stem is rubber...you can use either brass or nickel . The rarely leak...don't leak i have seen one pin leak in my 20 years.
the valve stem themselves leak at the rim and the beadseat of the tire are prone to leak.

you can check the tire yourself. pump the tire up to max inflation pressure listed on the tire sidewall....mix a solution of dishsoap and water in a spray bottle. spray the beadseat of the tire on both sides, valve stem with cap off and the tread area....let sit for 5min and look for bubbles.

 

It was the valve stem core as you said. Since they came in a pkg of 4 with the tool, I replaced them all, aired the tires back up, and all is well. TYTY