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The tire pressure monitoring is different in the US than it is in some (most?) other countries. The US uses a Direct Tire Pressure Monitoring System (dTPMS) sensor inside the wheel that detects a change in tire air pressure. You can tell that your X3 has them from the metal valve stems on your wheels. The system needs to be initialized when you add or remove air from your tires, when a tire is replaced, or when you switch to winter or summer wheels.
Other countries use an Indirect TPMS system (iTPMS). Instead of directly measuring air pressure in the tire they measure rotational speed of the wheel. If a tire has lower air pressure then it will be slightly smaller in size, and will rotate at a higher speed.
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