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Q1: Where is the tire pressure sensor (usually) located ?
Q2: How does it connect to the ECU, I see no electrical connection going to the wheel?
Q3: Why do dashboards not tell you specifically which tire has low pressure rather than one light for all the tires?
Q4: Do most vehicle's factor the spare when checking tire pressure or only the active four?
There are two basic systems that monitor tire pressure. There are active and passive systems. The active systems use a sensor and transmitter inside the tire. Older systems attached the sensor to the wheel with a band clamp inside the tire, newer sensors are part of the valve stem and are again inside the tire. The sensors communicate wirelessly to the control module. More complex systems assign an ID sequence to each sensor and knows each location after programing. This means after rotation they must be relearned to their current location. Less complex systems just indicate that one of the tires have lower than normal pressure. Passive systems use the wheel speed sensor to calculate the difference in wheel speed of a low pressure tire. I can only guess that with this system four underinflated tire might not activate the warning system. Most systems do not monitor the spare. Some newer vehicles also include trailer tire monitoring as an option. There are also standalone systems that communicate wirelessly to a separate receiver in the tow vehicle.
