The Trainer #39: The nagging TPMS light

“The light wasn’t on when I got here!”

Your customer is adamant that the funny looking yellow warning light now brightly illuminated on his instrument panel was not on when he brought the car in. You know, the one that looks like a tire cutaway with an exclamation point in the middle of it? That’s the warning light for the Tire Pressure Monitoring System (TPMS), and if you’ve recently performed some type of tire-related service or repair, odds are you forgot to tell the TPMS controller that you made some changes.

The first TPMS systems were a bit more forgiving than their more modern cousins, relying on existing Antilock Braking System (ABS) sensors to monitor tire pressure. These early systems inferred the tire pressure by plugging wheel speed data into a complex algorithm to monitor tire circumference. A low tire has a smaller circumference and will rotate faster than a properly inflated tire of the same type and size. These systems had to be placed in a “learn” mode whenever tire rotations or replacements were performed. Tire pressures would be corrected and then the vehicle would be driven in order for the system to establish an accurate baseline.

More common today, though, are systems using pressure sensors (mounted by band to the wheel or as a part of the tire valve stem assembly). On many cars and light trucks, the sensors are assigned to a specific wheel position. Rotating the tires without relearning the sensor positions will result in incorrect displays and could cloud troubleshooting efforts if the TPMS warning illuminates for a valid reason. Imaging trying to find a leak in the wrong tire.

In the March 2015 edition of The Trainer, we’ll show you how to make sure you don’t have TPMS warning light comebacks, how to identify a vehicle with a TPMS system, and how to troubleshoot an illuminated TPMS warning light.