The TPMS sensors look like this:
If you realize that long part is a valve stem, it should be pretty obvious how this is installed inside the rim. See that number on the right hand side? That's the address that tells the body control module how to talk to that sensor. The BCM only stores one set of sensor addresses at a time and doesn't store old ones if you replace your TPMS sensors.
When you first get one of these cars new, there's one of these in each full sized rim/tire. If you install aftermarket rims, you have several options:
1. You can just ignore them, in which case you'll get a TMPS light on the dash. Thank the US federal government for that.
2. You can swap your original TPMS sensors to the new rims. This requires your tires to be removed temporarily from your old rims to retrieve them. Once the new rims rotate a few times, the car will recognize its old friends still programmed into the body control module.
3. You can get another set of TPMS sensors (new or used) and have them programmed to the car in place of the originals, leaving your original sensors in the original rims. Most tire shops can program in the new addresses when they install the tires, or you can do it yourself with something like an ATEQ Quickset tool. Techstream is also alleged to be able to do it, but I haven't used it myself.
However, to do that, you or the installer needs those numbers on the sensors to be able to address them in your body control module. If you buy a set of rims and tires that already have the sensors inside them, but you don't have the numbers, the installer has to pull the tires off to get that number. The car can't just "detect" those numbers if they're not already in its memory. If you just install rims and sensors without addressing them, you'll get the federal government light on your dash. Thanks Obama.
Yes. I would definitely ask that question. It would be helpful for you to know what he did.
Are you getting the original rims and tires with the car? If the original sensors are still in them, ask him if he has the numbers of those original sensors. If he does, they can be programmed in without having to take the tires off the rims.
Some of us use something like that ATEQ Quickset tool I mentioned above to program the TPMS system ourselves. It will store four sets of TPMS sensors in it. When I put my rims on my car, I got a second set of sensors and stored both sets in the ATEQ so I can flash them back quickly if I ever need to swap back to the stock rims. If you're getting the original rims and tires and the seller used something like that to program them himself, he may have the addresses still stored in the tool.
If you want your own ATEQ, they're about $120 new, but occasionally they pop up on Ebay for $50.
Might be. None of it is really as complicated as it seems. It's just a mystery to most people because most people don't mount their own tires and never even see the sensors.