Malfunction Junction - What's the TPMS Function?

Dear Bree,
I do not understand what the heck just happened with my tire.  Please translate.
I was driving into my neighborhood when my tire pressure light went on, and then BA-BAM!  My tire just went flat!  I’m talking about going from, “Hey, your tire pressure is low,” to dead tire.  Not wounded, not a little leak--FLAT!  Then, as if that wasn’t crazy and scary enough, when I got it to a tire place to repair, they told me that the tire sensor valve stem had corroded, so the sensor just went *POOF*, detached and fell into my tire—hence the instant flat! What?!  I don’t even know what a tire sensor valve stem is!
I think that I should have had some clue that this problem existed or might occur or something, but I didn’t.  I just assumed that when my tire pressure warning light lit up, I’d have a minute to do something about it.  Can you please explain to me what went wrong and what I can do about it in the future?  Anything else I ought to know about these sensor things?
In the Dark in Indiana

Dear In the Dark,
You’re not alone in your confusion.  We see this kind of thing in some of our locations (namely, the ones located in colder climates), but lots of people are unfamiliar with the problem.  Let me break it down for you.
I’ll start with a little history lesson.  The Great Firestone Recall of the late 90’s had a big influence on the movement toward TPMS (tire-pressure monitoring systems) becoming standard on new cars.  More than a hundred deaths were linked to rollovers caused by tread separation on these Firestone tires, and this led to the creation of the TREAD Act by the Clinton administration.  In short, it requires that all new cars sold after September 1^st, 2007, had to come equipped with TPMS that would alert drivers when their tires were under-inflated.  The benefits are two-fold: reduce the number of accidents on roadways and improve fuel efficiency. Now, whether or not you agree with a government mandate on this topic can be discussed at your dinner table; the point is that TPMS’s are here to stay; so, as consumers and drivers, we need to know as much as we can about them.
TPMS’s are designed to alert drivers when their tire pressure falls below 25% of the recommended cold tire-inflation pressure, and it does this by using pressure sensors in each wheel to directly measure the pressure in each tire (this is the direct tire-pressure monitoring system).  Another option is the indirect TPMS which uses the vehicle’s antilock braking system’s wheel speed sensors to compare the rotational speed of one tire against the others. If a tire is low on pressure, it will roll at a different number of revolutions per mile than the other three and alert the vehicle’s onboard computer.  Most people seem to agree that direct systems are more accurate and reliable, and also have the advantage of usually being able to identify which tire is having trouble.
But TPMS’s are far from perfect.  The most common problem seems to be valve stem corrosion.  If a brass valve core is installed into an aluminum stem or a brass cap is screwed onto an aluminum stem, you’ve got a problem.  These two metals don’t mix; put them together and you get galvanic corrosion, especially if you’re living with Nanook of the North and dealing with ice melting chemicals and salt on roadways.  If this corrosion occurs, these two dissimilar metals will fuse together and cause damage to the stem, core and cap, and eventually the sensor itself.   When a dealer or tire repair professional goes to remove the cap or valve core, or just check the tire pressure, the whole sensor just breaks off or the valve crumbles.  Not good, right?  Sounds like this might be what happened to you.
Another common issue is that eventually, the batteries in these units die (anywhere between 3 and 10 years is the life span); and when they die, the entire unit has to be replaced.  It is recommended that if you replace one TPMS, you replace them all at the same time (more than likely, the others are on their last gasps as well); and this, as you can imagine, can get pricey.  You may be facing a similar fate with the sensors on your car.  If one was this badly corroded, how are the others looking?
So, now you know.  Don’t mix the metals.  Ask questions.  In 2012, it was estimated that there were more than 200 million TPMS sensors on the road, and more than 35% of them were at least 3 years old, which means that in the next couple of years, more than 9 million sensors will need to be replaced.  That’s a lot of sensors.
We deal with problems like this all the time at RNR, so if you come see us, we can help you out and get you back on the road.  Take care!

Bree