How does TPMS know where each tire is?
Tire Pressure Monitoring Systems (TPMS) locate each tire’s position in one of two ways: using a dedicated sensor in every tire (direct TPMS) or by sensing wheel speeds with no tire-specific transmitters (indirect TPMS). Direct systems map sensor IDs to wheel positions, while indirect systems infer location through software and driving-based calibration. The exact steps depend on the vehicle make and model.
Direct TPMS: sensors, IDs, and positioning
Direct TPMS relies on a sensor in each tire that reports its unique ID together with real-time pressure (and often temperature). The vehicle’s TPMS controller stores a map that links each sensor ID to a specific wheel location. When tires are rotated, replaced, or the sensors are swapped, the system must relearn which ID belongs to which corner of the vehicle. Relearns can be automatic through driving or performed with a diagnostic tool or the vehicle’s onboard menus.
- Each tire sensor broadcasts a radio signal containing a unique ID and real-time pressure data to the TPMS ECU.
- The TPMS ECU maintains a position map that links each sensor ID to a wheel location (e.g., FR, FL, RR, RL).
- During a relearn or tire service, technicians use a scan tool (or vehicle menus) to pair the sensor IDs with the correct wheel positions, or the car may learn automatically while you drive.
- When a tire is replaced or a sensor is swapped, a new ID must be programmed and mapped to its position for accurate warnings.
- Some vehicles support automatic relearns by simply driving at a set speed for a period of time, while others require manual input to complete the mapping.
The exact steps vary by manufacturer and model, but the core idea is that a known ID-to-position mapping lets the car tell you which tire is underinflated and trigger the correct warning light.
Indirect TPMS: how it infers location without sensors
Indirect TPMS does not carry tire-specific sensors; instead, it uses the vehicle’s ABS wheel-speed sensors to monitor rotation rates and infer if a tire’s effective circumference has changed due to underinflation. Because there are no tire IDs, position mapping is not based on sensor IDs. Instead, the system may calibrate locations through software learning or by a user-initiated relearn after tire rotation or replacement. In many cars, you must drive or use a tool-driven process to re-map wheel positions so the warnings show the correct location.
- There are no tire-mounted transmitters in indirect TPMS; it relies on ABS wheel-speed sensors to detect anomalies in wheel circumference that indicate low pressure.
- The system typically does not know the exact tire identity or location unless a relearn procedure is performed or the software assigns positions based on the current configuration.
- After rotating tires or replacing them, many manufacturers require a relearn or a certain driving sequence to re-map wheel positions so the warnings show the correct location.
- Limitations: Indirect TPMS can be less precise, especially with mixed tire brands or irregular wear, but it avoids the cost of sensor-equipped tires.
While indirect TPMS provides a cost-effective alternative, direct TPMS delivers location-specific alerts with higher accuracy and quicker diagnostics, especially when inflations fluctuate due to temperature or altitude changes.
Summary
Direct TPMS uses sensor IDs and an ECU mapping to label each tire by position, with relearns required after service. Indirect TPMS uses ABS wheel-speed data to infer underinflation and may require software-driven or driving-based learning to assign positions. In practice, most modern vehicles support a relearn procedure to keep the system accurately aligned with wheel locations, ensuring the right tire location is reported when a warning occurs.
