What the light is telling you
The tire pressure warning light is the horseshoe-shaped symbol with an exclamation point inside. It comes on when a sensor reads a tire well below the recommended pressure. Many systems are set to warn when a tire is about a quarter below where it should be, so by the time the light shows, the tire is already noticeably low.
How temperature moves the number
Air pressure follows temperature. As a rule of thumb, a tire loses about 1 psi for every 10 degrees the air gets colder. North Texas is a place where a cold front can drop the temperature 30 degrees or more in a day, so a tire that sat a little low when it was warm can cross the warning line overnight. A tire at 35 psi on a 70-degree afternoon is closer to 32 on a 40-degree morning. That's three pounds with no leak at all.
That's why the light often comes on in the morning and goes out later. Driving heats the tires and raises the pressure a few pounds. The light going out doesn't mean the tire is fine. It means the weather helped.
What heat does
Heat does the opposite. A tire's pressure rises as it warms, which is normal and which the recommended pressure already allows for. The summer risk is a tire that's low to begin with. It flexes more, runs hotter and wears faster, and a hot freeway is where an underinflated tire fails.
So a summer light is more likely to mean a real leak or a tire that was neglected than a swing in the weather.
What to do
Check the pressure with a gauge while the tires are cold, either before you drive or after the car has sat for a few hours. Use the number on the sticker in the driver's door jamb, not the larger number molded into the sidewall, which is the tire's maximum. Add air to match the sticker.
A basic gauge costs very little and lives in the glove box. Gas station air pumps often have a gauge built in, but a pump that's been dragged around for years can be off, so trust your own. Check all four tires, not only the one the light seems to point at.
If the light is back within a day or two, or one tire keeps dropping, the tire is leaking. A screw or nail in the tread is the usual cause. TRA removes it and makes the tire safe for $70, and repairs a tread puncture for $54–$60, each plus the $19 dispatch fee charged once per job. If a light flashes for about a minute and then stays on, the system itself may be at fault, and that's a question for a shop.
Common questions
Check the tires first. If one is visibly low or flat, don't drive on it. If they're only a little low, fill them to the door jamb number at the next stop.
Some systems take a few miles of driving to reset. If it stays on after that, a tire may still be low, or one of the sensors may need a shop to look at it.
Once a month, and after a big temperature change. It takes a couple of minutes with a gauge and is the cheapest tire maintenance there is.