TPMS: why your tire pressure light is on and what to do about it
Understanding direct vs. indirect TPMS, seasonal pressure changes, and when a TPMS warning means more than just low air.
The TPMS (Tire Pressure Monitoring System) light — a horseshoe shape with an exclamation mark — illuminates when any tire drops 25% or more below the recommended pressure. On most cars, the recommended pressure is 32-36 PSI (check the door jamb sticker, not the tire sidewall).
The most common trigger is seasonal temperature change. Tire pressure drops about 1 PSI for every 10-degree Fahrenheit drop in ambient temperature. A tire inflated to 35 PSI in summer can read 28 PSI on the first cold morning of fall — enough to trigger the light. The fix is simple: add air to the recommended pressure at any gas station.
There are two TPMS types. Direct TPMS uses sensors inside each tire that transmit actual pressure readings to the car's computer. Indirect TPMS uses the ABS wheel speed sensors to detect a tire spinning faster than the others (an underinflated tire has a smaller circumference and spins faster). Direct systems are more accurate but the sensors cost $50-80 each and have batteries that last 7-10 years.
If the TPMS light flashes for 60-90 seconds and then stays solid, the system itself has a fault — usually a dead sensor battery. If the light is solid, at least one tire is genuinely low. Never ignore TPMS: driving on a significantly underinflated tire generates heat that can cause a blowout at highway speeds.
Information is for educational purposes only. Always consult a qualified mechanic for diagnosis and repair. AutoAt's knowledge base is AI-assisted and regularly updated but may not reflect the most current manufacturer specifications. Always verify critical specifications with official service manuals. Repair procedures described here may require professional tools, training, and certifications. Attempting repairs beyond your skill level can be dangerous.