Originally posted by WPRESTO Remote tire pressure gauges are definitely double-edged. On the one hand they warn you when a tire is soft, a condition that can creep up slowly so that you don't notice the gradual changes in the car's handling. On the other, they operate on a battery independently of the car's electrical system. If the battery goes bad, as best I can tell, the warning light is activated and remains on until the battery, or sometimes the entire sensing unit is replaced so that you aren't driving with a non-functioning sensor and would not be warned if that tire went soft. BUT, which wheel has the detector with a dead battery? At least in cars I've driven, when the soft tire light comes on, you can only determine which tire has the problem is to go around and test each one with a pressure gauge. If one pressure gauge has a dead battery, all the tires have correct pressure, and SFAIK the only way to find it is to remove each tire from its rim so you can access the gauge and test its battery. If I'm wrong about this and there is some way to locate a pressure gauge without getting your hands on it, please alleviate my ignorance.
The newer vehicles with TPMS systems will identify each sensor bit its unique serial code, via the wireless link to the ECM. The information display should identify which tire has low air pressure. It should also identify which sensor has a low or dead battery.
If the tires are rotated during routine service, the mechanic needs to reset the position of the sensors.
Yes, to get at the sensor for repair or replacement the tire must be dismounted from the wheel.
Also, if one sensor battery goes bad, all the sensor batteries should be replaced. They are all the same age, so only replacing them as they fail will have you visiting the repair shop more times than you would like.