The tire pressure light on your dashboard exists because of a deadly tire scandal

That little horseshoe-shaped light with an exclamation point in it isn't a nice-to-have feature. It's required by federal law, and the law exists because of one of the deadliest product scandals in American history.

Tires coming apart

In the late 1990s, a pattern started showing up in crash reports. Ford Explorers, then one of the best-selling vehicles in the country, were rolling over after their Firestone tires failed at highway speed. The tread was peeling away from the body of the tire — a failure called tread separation — often in hot climates and at high speeds.

The crashes were brutal because an SUV losing a tire at 70 mph can easily roll. At least 238 people were killed and around 500 injured in the U.S., with more deaths in other countries.

The recall

In August 2000, Firestone recalled about 6.5 million tires. Then the two companies turned on each other in public.

A big part of the fight was about tire pressure. Ford had recommended inflating the Explorer's tires to 26 psi, on the low side. Firestone argued it should have been 30 psi, and that underinflation made the tires run hotter and fail. Ford blamed the tires. Within a year, the nearly century-old business relationship between the two companies was over.

The law

Congress held hearings and passed the TREAD Act, signed in November 2000. Among other things, it required carmakers to report problems to federal regulators faster — and it led to a requirement that new cars come with a tire pressure monitoring system (TPMS).

The rule phased in starting in 2005, and since September 2007, every new light vehicle sold in the U.S. has had to have one.

The catch most people don't know

The federal rule only requires the light to come on when a tire is 25% or more below the recommended pressure. On a tire that should be at 35 psi, that's about 26 psi. By the time the light comes on, the tire is already seriously low — not just a little off.

What this means for you

• Don't wait for the light. Check your pressure about once a month, with the tires cold.
• Use the number on the driver's door sticker, not the one on the tire. Why the tire's number is the wrong one.
• If the light comes on, check all four tires soon. If it flashes and then stays on, the sensor system itself may have a problem.
• Check your tread too. The penny test is later than you think.

Quick reference

• Firestone tires on Ford Explorers were linked to at least 238 U.S. deaths
• 6.5 million tires were recalled in August 2000
• The TREAD Act led to tire pressure monitors in every new light vehicle since September 2007
• The light only has to come on at 25% below the correct pressure — that's already very low
• Check pressure monthly using the door-jamb sticker number