When Does the Tire Pressure Light Come On?
3 lessons in this series, 7 min in total
- What PSI Should Your Tires Be? Read the Door-Jamb Label 3 min read
- When Does the Tire Pressure Light Come On? 2 min read
- How to Fill Your Tires With a Portable Inflator 2 min read
This lesson draws on the federal TPMS standard FMVSS No. 138, the U.S. Tire Manufacturers Association (USTMA), Continental and Kelley Blue Book’s report on AAA’s TPMS study. See how we research.
The 25% rule in FMVSS No. 138
FMVSS No. 138 requires a tire pressure monitoring system (TPMS) on vehicles rated at 10,000 lb (4,536 kg) gross vehicle weight or less, with some exceptions. The light must come on within 20 minutes once one or more tires drop 25% below the carmaker’s recommended cold pressure. For standard P-metric tires, the trigger can’t be lower than 20 PSI (1.4 bar).
The standard also requires a fault warning: on cars with one combined light, the light flashes for 60 to 90 seconds, then stays on. USTMA says the low-pressure warning may come too late to prevent damage on many vehicles.
| What | Number | Source |
|---|---|---|
| Legal trigger | 25% below placard cold pressure | FMVSS No. 138 |
| Lowest trigger, P-metric tires | 20 PSI (1.4 bar) | FMVSS No. 138 |
| Time to warn | Within 20 minutes | FMVSS No. 138 |
| 35 PSI (2.4 bar) placard | Warns at about 26 PSI (1.8 bar) | Our calculation |
| AAA trigger points | 5 of 11 at 75% of posted pressure; 5 of the other 6 at 70.3-72.9% | AAA via KBB |
| TPMS reading accuracy | Within 1.2-1.5% of a calibrated gauge on average | AAA via KBB |
Why it comes on on cold mornings
Temperature moves the reading even when nothing leaks. Continental puts the change at about 1 PSI (0.07 bar) for every 10°F (5.6°C). USTMA gives 1 to 2 pounds (0.07-0.14 bar) for every 10 degrees of temperature change.
By Continental’s rule of thumb, a tire set to 35 PSI (2.4 bar) at 75°F (24°C) would read about 31.5 PSI (2.2 bar) at 40°F (4°C), with no leak at all. That drop alone can push a tire that was already a little low past the warning point overnight.
Kelley Blue Book’s report on AAA’s study gives the same advice: check regularly, especially in cold weather. If the light comes on after a cold night, fill to the placard pressure while the tires are still cold.
What AAA’s test found
AAA tested 11 vehicles from model years 2022-2024, according to Kelley Blue Book; the last two table rows show where their lights came on and how accurate their readings were. Even so, AAA still recommends a manual check about once a month.
If you plan to use an inflator’s built-in display as your gauge, first read how accurate portable tire inflator gauges are.
Why is my tire pressure light still on after filling?
The Autopian notes that some cars need a manual TPMS reset after you correct the pressure. Your owner’s manual has the steps.
When a tire reads low, here’s how to fill your tires with a portable inflator.
