TPMS — Tire Pressure Monitoring System — has been required on every car sold in the U.S. since 2008. The yellow exclamation-point light has saved a lot of tires, but it also confuses a lot of drivers. Here's how it actually works.
Direct TPMS
The more accurate type. Small battery-powered sensors live inside each wheel, attached to the valve stem. They measure actual tire pressure in PSI and transmit it wirelessly to the car's computer. The dash light triggers when any tire drops 25% below the door-jamb specification.
Direct TPMS is used by most American manufacturers. It's accurate, gives some cars an actual PSI readout on the dash, and works in any condition.
Indirect TPMS
Used by some European and Asian makes. It doesn't measure pressure directly — it uses the ABS wheel-speed sensors to detect when one tire is rolling slightly faster than the others (which happens when a tire is underinflated and has a smaller effective rolling diameter).
Indirect TPMS is cheaper (no sensors), but it's less accurate, doesn't show a PSI number, and needs to be reset every time you change tires or adjust pressures.
What the light means
Solid yellow exclamation point: one or more tires is at least 25% under-pressure. Check all four pressures cold, top off to the door-jamb spec, and the light should go out within a few miles of driving.
Flashing yellow exclamation point: TPMS system has a fault — a sensor battery is dead, or the system isn't reading. The light usually flashes for 60-90 seconds at startup, then goes solid. You'll need to diagnose which sensor or whether the system itself needs service.
When sensor batteries die
Direct TPMS sensors have internal batteries that last 5-10 years. When they die, the sensor must be replaced — the battery is sealed inside. A new sensor costs $40-80 each, plus mount/balance labor. Plan to replace at least some on cars over 10 years old.
Resetting after tire changes
When you swap winter and summer tire sets on the same car, each set should have its own TPMS sensors. After mounting, the car needs to learn the new sensor IDs. Most cars do this automatically after about 15-20 minutes of driving above 20 mph. Some require a manual reset through the dash menu or with a TPMS programming tool at the tire shop.
Why ignoring TPMS is expensive
A tire driven 50 miles at 25% under-pressure can sustain hidden damage to the sidewall and inner liner that doesn't show up for weeks. The tire may fail catastrophically later. Plus, you're losing 1-3% fuel economy per 5 PSI low. The light costs nothing to address; ignoring it costs a tire.
TPMS isn't a suggestion. It's the cheapest insurance on the car.