How do tire pressure sensors know which tire is low?
Direct tire pressure monitoring systems (TPMS) identify the exact tire that is underinflated by using a unique sensor ID attached to each tire, while indirect TPMS infers underinflation from wheel-speed data rather to pinpoint a single tire. This article explains how each approach works and what drivers should know about locating and addressing a low tire.
Direct TPMS: A Sensor per Tire
Direct TPMS relies on individual sensors mounted in or on each tire to report precise pressure data along with a unique identification code to the vehicle computer. The system uses a position map to link sensor IDs to tire locations, allowing it to tell you exactly which tire is low.
- Each tire carries a dedicated TPMS sensor (often in the valve stem or mounted to the rim) that broadcasts a unique ID and the tire’s pressure (and sometimes temperature).
- The TPMS control module maintains a mapping from sensor IDs to wheel positions (for example, front-left, front-right, rear-left, rear-right).
- When a tire’s pressure falls below the preset threshold, the corresponding sensor sends a low-pressure signal; the ECU uses the sensor ID to identify the specific tire and triggers the warning with a location cue.
- After tires are rotated, remounted, or sensors are replaced, a relearn or initialization procedure is usually required to re-link sensor IDs to the correct positions. This can involve driving at a certain speed, performing a manual relearn in the vehicle’s settings, or using a diagnostic tool.
- Many modern vehicles display the precise tire location after the relearn is completed; some require using the manufacturer’s app or onboard system to view the location.
In short, direct TPMS ties a tire’s unique sensor ID to a fixed position on the vehicle, so when the tire is low, the system points you to the exact corner that needs attention.
Indirect TPMS: Inferring the Low Tire
Indirect TPMS does not measure air pressure in each tire. Instead, it uses the vehicle’s ABS/ESP wheel-speed sensors and expected tire size to detect anomalies in rolling circumference. It can indicate that a tire may be underinflated, but it often cannot reliably tell you which tire is low.
- The system monitors differences in wheel speeds and rotation that arise when a tire’s pressure is lower, changing its effective diameter.
- Using ABS/ESP wheel-speed data and predefined tire size expectations, the algorithm flags a potential underinflation condition.
- The warning may be generic (e.g., “low tire”) without a specific tire location, or it may use vehicle dynamics data to estimate the likely tire location in some newer implementations.
- Indirect TPMS can be more prone to false positives under certain conditions (heavy loads, dynamic driving, road surfaces, or wear), and it may require calibration or a tire change/relearn to improve accuracy.
Indirect TPMS provides a helpful alert without needing sensor hardware in every tire, but locating the exact tire often requires a direct pressure check with a gauge to confirm the culprit.
What to Do When TPMS Warns
When the TPMS light comes on, follow these general steps to verify and address the issue. The steps differ slightly depending on whether your vehicle uses direct or indirect TPMS.
- Check all tire pressures with a gauge and compare to the vehicle’s recommended cold-inflation pressure printed on the driver’s door jamb placard or in the owner’s manual; inflate tires that are underinflated to spec.
- Inspect tires for damage, nails, punctures, or sidewall leaks; look for signs of rapid deflation or a tire that feels soft or bulges.
- If the warning persists after correcting pressures, consult a tire service professional. Direct TPMS may require sensor repair or a relearn procedure; indirect TPMS may need calibration after tire work.
- Consider rotating tires or replacing a failed sensor if a particular tire continues to report anomalies; keep an eye on the system after any service to confirm it’s functioning correctly.
By verifying pressure and inspecting tires, drivers can ensure safety and restore accurate TPMS readings. Relearn procedures are essential after tire service or rotation to maintain correct location mapping in direct TPMS systems.
Summary
Direct TPMS provides precise tire-level identification by using individual sensors that broadcast a unique ID and pressure, paired with a position map in the vehicle. Indirect TPMS detects underinflation by analyzing wheel-speed data and rolling circumference, without per-tire IDs, which can make locating the exact tire less precise. After service, many systems require a relearn to re-map sensor IDs to tire positions. Regularly checking tire pressure with a gauge and following manufacturer procedures helps keep TPMS accurate and ensures safe driving.
