TPMS: what it does and why it matters

A tire pressure monitoring system (TPMS) warns you when a tire has lost enough air to be unsafe. New vehicles sold in the United States have needed one since the 2008 model year. Canada does not require it, but most vehicles sold here have it anyway.

Direct and indirect systems

Direct TPMS has a sensor inside each wheel that measures the air pressure and sends it to the vehicle. It usually warns at about 25 percent below the recommended pressure, and many vehicles show the pressure in each tire.

Indirect TPMS has no pressure sensors. It uses the ABS wheel speed sensors to notice a tire turning faster than the others, which is what a low tire does. It is less precise and cannot tell you the actual pressure.

Winter wheels without sensors

If your winter tires are on a second set of wheels with no sensors, the warning light will stay on all winter. On most vehicles nothing else is affected, because the system works on its own. You lose the low-pressure warning, so check the pressures yourself.

The alternative is a set of sensors for the winter wheels. Ask at the counter what your vehicle needs.

What the light is telling you

• On and steady: at least one tire is low. Check all four and top up.
• Flashes, then stays on: the system itself has a fault and is not watching your tires. Have it looked at.
• Keeps coming back: there may be a slow leak. Bring it in.

Still check your pressures

A TPMS warning means a tire is already well below where it should be. Checking once a month catches a slow loss early, saves fuel and makes tires last longer.