Troubleshooting why the TPMS light is on

Why the TPMS Light Is On — Understanding Tire Pressure Monitoring Systems

The Tire Pressure Monitoring System (TPMS) is an essential feature in modern vehicles, designed to alert drivers when their tire pressure is too low. While it’s a helpful tool for maintaining safety and efficiency, it can also be a source of confusion and concern when the TPMS light illuminates your dashboard. This article will delve into the reasons why your TPMS light is on and what steps you can take to address it.

What is TPMS?

TPMS stands for Tire Pressure Monitoring System, a system that monitors the air pressure inside pneumatic tires on various types of vehicles. The primary function of TPMS is to alert the driver when a tire is significantly under-inflated, which can lead to unsafe driving conditions.

Types of TPMS

• Direct TPMS— This system uses sensors located in each wheel to measure tire pressure directly. These sensors send real-time data to the vehicle’s onboard computer.
•Indirect TPMS— Instead of direct pressure measurements, this system uses the vehicle’s ABS (Anti-lock Braking System) wheel speed sensors to detect variations in wheel speed that could indicate a low tire.

Common Reasons Why the TPMS Light Is On

1. Low Tire Pressure— The most straightforward reason for the TPMS light is simply low tire pressure. Tires lose pressure over time, even without a puncture, due to the natural permeability of rubber. Changes in temperature can also affect tire pressure.

• Temperature Fluctuations— Large changes in outdoor temperature can affect tire pressure. Every 10°F in outside temperature chances tire pressure by 1° psi. So, if a cold front rolls in overnight and the temperature drops by 40°F, your tire pressure will drop by 4 psi. If your tire was already low on pressure, the drop caused by the cold front could activate the TPMS system and that’s why the TPMS light is on. This is why you might notice the TPMS light more frequently during significant temperature changes. Conversely, the same 10°F increase in outside temperature can raise air pressure in your tires and turn off the TPMS light.

2. Punctures or Leaks— A more serious cause for why the TPMS light is on could be a puncture or leak in one of your tires. This could be due to driving over a nail, glass, or other sharp objects that compromise the tire’s integrity.

• Slow Leaks— Sometimes, the puncture might be small, causing a slow leak that gradually reduces tire pressure. This slight decrease can trigger the TPMS light.

3. Faulty TPMS Sensor— If the TPMS light is on but you’ve checked the pressure in all your tires and they’re at the proper pressure, then there might be an issue with one of the sensors.

• Sensor Battery Life— TPMS sensors, especially in direct systems, are battery-operated and have a finite lifespan, usually around 5-10 years. If the battery dies, the sensor can no longer send accurate readings to the vehicle’s computer.
• Sensor Damage— Sensors can also be damaged by corrosion, impact, or during tire changes if not handled properly.

How TPMS sensors work

The battery is set into the sensor using “potting” epoxy. This is done to protect against moisture intrusion and corrosion. However, the battery can’t be replaced without destroying the sensor. If the battery dies, the entire sensor must be replaced.

The TPMS sensors are usually in sleep mode. If the vehicle isn’t moving, the transmitter sends a low-power signal once each hour. It wakes up when the vehicle is in motion and sends a brief report to the TPMS receiver every 30 to 60 seconds once the vehicle reaches 15-20 MPH.

Why TPMS is required

Low tire pressure increases rolling resistance which decreases fuel mileage. Worse yet, low tire pressure causes excessive tread and sidewall flex which increases heat buildup. Heat buildup dramatically reduces the life of the rubber and internal belts and can cause premature and sudden tire failure. Studies show that nearly 250,000 accidents every year are caused by underinflated tires and that 75% of all roadside flats are preceded by a slow leak and under-inflation.

How to reset a TPMS system

If a tire is low and you’ve received a dashboard notice, fill the tire with air and drive the vehicle. After a while, the sensor will wake up and report to the receiver and the warning light should go out if it doesn’t refer to your owner’s manual for instructions on how to perform a manual reset.

Be aware that some carmakers install TPMS sensors on spare tires. So, if you’ve filled your four driving tires and still have a TPMS light on, double-check the pressure in the spare tire before assuming you’ve got a problem.

Other things to know about TPMS sensors

On some first-generation TPMS sensors, the metal valve stem cap is part of the radio antenna. If you lose the cap, the TPMS sensor may lose contact with the receiver.

Early sensors were made with different metals, which was a huge mistake on the part of the carmakers. As a result, galvanic corrosion can set it and eat away at the metal portions of the valve stem. If you try to remove the metal cap, you can actually break off the valve stem. That’s why many shops warn you they won’t be responsible for broken TPMS sensors.

TPMS sensor battery life is 7-10 years and the battery is not replaceable. When the battery fails, you must replace the entire TPMS sensor.

You do not need to go to the dealer for a new TPMS sensor. Any tire shop can replace and program a TPMS sensor for far less than a dealer.

Shops are prohibited by law from disabling a TPMS system. Don’t ask them to disable your TPMS system.

©, 2020 Rick Muscoplat