TPMS Systems

The Tire Pressure Monitoring System — TPMS — is a federally mandated safety system on all vehicles sold in the US after 2007. Its job is simple: warn the driver when a tire drops below a safe pressure. The TPMS light on the dash means at least one tire is 25 percent or more below the placard pressure. That is significant — a tire rated at 35 PSI triggering at around 26 PSI is already dangerously low and generating excess heat.

Direct systems use a physical sensor mounted inside each tire, usually attached to the valve stem or banded to the inside of the wheel. Each sensor contains a pressure transducer, a temperature sensor, a battery, and a small radio transmitter. The sensor reads the actual tire pressure and wirelessly transmits that data to a receiver module — usually the BCM or a dedicated TPMS module. The system knows the exact pressure in each tire and can display individual tire pressures on some vehicles. When any sensor reads below the threshold, the TPMS warning light illuminates.

Indirect systems do not use pressure sensors inside the tires. Instead, they use the existing ABS wheel speed sensors to detect pressure loss. The principle is simple — a tire with lower pressure has a slightly smaller rolling diameter and rotates faster than the other tires. The ABS module compares the rotational speeds of all four wheels. If one wheel consistently spins faster than the others, the system flags it as low pressure. Indirect TPMS is less precise than direct. It cannot tell you the actual pressure — only that one tire is significantly lower than the others. It also cannot detect a situation where all four tires are equally low.

Direct TPMS sensors run on a small lithium battery sealed inside the sensor housing. The battery is not replaceable — when it dies, you replace the entire sensor. Battery life is typically 7 to 10 years depending on the sensor design and how frequently it transmits. As sensors age, the battery weakens and transmission range decreases. A sensor with a dying battery may intermittently fail to communicate with the module, causing the TPMS light to flash for 60 to 90 seconds at startup — which indicates a system fault, not a low tire — and then stay solid. When one sensor battery dies, the others are usually close behind because they were all installed at the same time.

Each TPMS sensor has a unique ID. The TPMS module must know which sensor ID belongs to which wheel position — left front, right front, left rear, right rear. Any time tires are rotated, replaced, or sensors are replaced, the system needs to relearn which sensor is where. Some vehicles use a scan tool to program each sensor ID to a position. Some use a TPMS activation tool — a handheld device that sends a signal to wake up each sensor one at a time while the module listens. Some vehicles self-learn by driving at a specified speed for a specified distance after a reset. Always look up the specific relearn procedure for the vehicle. Skipping the relearn means the dash may display the wrong pressure for each tire position or the TPMS light may stay on.

Solid TPMS light — at least one tire is below the pressure threshold. Check and correct all tire pressures to the door placard specification. The light should turn off after a few minutes of driving. If it does not, one tire may have a slow leak or the sensor may be reading incorrectly. Flashing TPMS light at startup that then turns solid — this indicates a system fault, not a tire pressure issue. A sensor battery has died, a sensor is not communicating, or the relearn was not performed after a tire service. Scan the TPMS module for codes to identify which sensor is faulting.