The low-tire-pressure light on your dashboard is not a courtesy feature — it is mandated equipment with a federally defined trigger. The standard behind it is Federal Motor Vehicle Safety Standard No. 138, codified at 49 CFR 571.138, which the rule itself describes as specifying "performance requirements for tire pressure monitoring systems (TPMSs) to warn drivers of significant under-inflation of tires and the resulting safety problems." The standard applies to passenger cars, multipurpose passenger vehicles, trucks, and buses with a gross vehicle weight rating of 4,536 kilograms (10,000 pounds) or less — the light-vehicle fleet — with an exception for vehicles that have dual wheels on an axle.

FMVSS 138 begins by fixing what a TPMS actually is, in a definition that is narrower and more precise than the everyday phrase "tire sensor."

"Tire pressure monitoring system means a system that detects when one or more of a vehicle's tires is significantly under-inflated and illuminates a low tire pressure warning telltale."— FMVSS No. 138 (49 CFR 571.138), source

The threshold and the timing

The two numbers that define a compliant TPMS are the under-inflation threshold and the warning window. Under the standard's requirements, the system must illuminate the low-tire-pressure warning telltale not more than 20 minutes after the inflation pressure in one or more of the vehicle's tires — up to a total of four tires — is equal to or less than the pressure 25 percent below the vehicle manufacturer's recommended cold inflation pressure (or a pressure specified by a separate column in the standard, whichever is higher). Those two figures together are the heart of FMVSS 138: a 25 percent under-inflation trigger and a 20-minute warning deadline. A tire that drops a few psi will not trip the light; the standard is aimed at significant under-inflation, the kind associated with handling loss, heat build-up, and tread separation.

The reference point for "recommended cold inflation pressure" is itself a regulated artifact. FMVSS 138 ties its threshold to the Vehicle Placard and the tire inflation pressure label — the sources of the manufacturer's recommended cold inflation pressure under 49 CFR 571.110. That linkage matters because the 25 percent threshold is measured against the manufacturer's spec for that specific vehicle, not a generic number; the placard on the driver's door jamb is the baseline the TPMS judges against. The standard also distinguishes the warning telltale's behavior — it must illuminate and, by design, remain illuminated while the low-pressure condition persists — from a momentary alert.

What the standard does and does not promise

It helps to be precise about what FMVSS 138 guarantees. It guarantees that a covered light vehicle carries a system that will detect significant under-inflation in up to four tires and warn the driver within 20 minutes of crossing the 25 percent threshold. It does not require the system to display the exact pressure of each tire, and it does not require detection of slow leaks before they reach the threshold — many vehicles offer per-tire pressure readouts, but that is a manufacturer feature beyond the federal floor. The standard sets a minimum capability: a binary warning tied to a defined under-inflation level, delivered within a defined time.

The reason the standard exists is the safety chain that significant under-inflation sets off. An under-inflated tire flexes more, runs hotter, wears faster, and is more prone to failure at speed — the failure mode behind a wave of crashes that prompted the statutory mandate for tire-pressure monitoring in the first place. FMVSS 138 converts that mandate into a measurable requirement: a defined trigger (25 percent below the placard pressure), a defined response time (20 minutes), and a defined indicator (the low-pressure telltale). When the light comes on, it is reporting that at least one tire has reached the federally specified under-inflation level — and the document that says exactly what that means is 49 CFR 571.138.

Direct versus indirect TPMS, and why it matters

FMVSS 138 is a performance standard, which means it specifies what the system must achieve, not how it must work — and manufacturers meet it with two distinct technical approaches. A direct TPMS places a pressure sensor inside each wheel that measures actual tire pressure and transmits it to the vehicle. An indirect TPMS infers under-inflation without a pressure sensor, typically by reading wheel-speed data from the antilock-braking system: an under-inflated tire has a slightly smaller rolling radius and turns marginally faster, and the system flags the discrepancy. Both can be engineered to meet the standard's 25-percent trigger and 20-minute warning, which is why FMVSS 138 deliberately avoids prescribing a sensor architecture. The practical consequence for owners is that an indirect system generally must be reset after tire rotation or reinflation so it can relearn the baseline, and it cannot tell you which specific tire is low — capabilities a direct system provides as a side effect of measuring real pressure.

The standard also accounts for the warning's persistence and for malfunction. The low-pressure telltale is required to stay illuminated as long as the under-inflation condition exists, rather than flashing once and clearing, so a driver who misses the initial alert still sees the warning on the next start. Separately, the standard addresses what happens when the TPMS itself fails — through a malfunction indication — so that a non-functioning monitoring system does not silently leave the driver believing the tires are being watched when they are not. That combination of a defined trigger, a bounded warning time, a persistent telltale, and a malfunction indication is what turns "tire pressure monitoring" from a marketing label into a federally measurable capability. The full set of requirements, and the columns and thresholds that govern the trigger, are spelled out in the codified standard at 49 CFR 571.138.