Remember all those people who bought new cars through the "Cash for Clunkers" program this summer?

They are now getting used to all the new gadgets on their new cars, including the Tire Pressure Monitoring System or TPMS.

The federal government mandates a TPMS on all vehicles built after 2007. Some car makers put the systems in their cars and trucks before that date.

Some systems use the ABS (Anti-lock Braking System) wheels speed sensors to detect an under-inflated tire: an under-inflated tire is "shorter" than a properly inflated tire and runs at a different speed.

But most new cars and trucks use a TPMS sensor inside the tire, connected to inside of the valve stem, which sends radio signals to the vehicle's computer. If a tire is under-inflated, a dash warning light tells you. This is a dash light that can't be ignored, since it usually means a tire has a leak, which can lead to a crash.

This is a picture of a typical TPMS sensor ... this one is from a Toyota.



You can see that it is not much bigger than a couple of quarters, but this little bit of technology costs $110 to replace. If you do need a new one, you also have to pay another $30 to have the vehicle's computer programmed to work with a new sensor.

Why should you care? Because some day your new car is going to need new tires, and there is a chance that the shop that mounts the new tires will ruin the TPMS sensor.

This is a picture of one those sensors after a tire shop couldn't get it to separate from the wheel.



What you need to know is that SOME shops leave the sensors mounted to the wheels and carefully install the new tires ... while OTHER shops believe the sensor should be disconnected from the wheel and dropped into the tire to avoid tire machine damage. (A tire machine is used to put tires on and take tires off the wheels.)

Before you get new tires (or a flat tire repair) on a TPMS-equipped vehicle, you should discuss with the mechanic whether they will pay for the cost of a damaged sensor. Check with the dealership as well as a tire shop.

If "Billy Bob's Tires" has the best price on new tires but ruins a TPMS sensor, you will spend more than you planned. If a dealership charges more for tires but replaces a damaged sensor for free, it might be the better deal.

The important thing is to have the discussion BEFORE any work is done.