Rberq,
Yes there is one connector on all cars built model year 1996 and later, it is called the obdII diagnostic connector. All cars after 96 must have this, and they must all communicate on the same protocol for emissions related failures. Most new cars are going to what they call CAN(controller area network) same connector but differant communication protocol(So depending on the car you would need a can compatible scanner). That is where the similarities end, some cars still have a factory connector somewhere on the car, that is where you get the good info for climate control problems, body control problems, electronic suspensions ect, but than you need a scan tool that can read that and the appropiate connector. So to answer a question some cars have two connectors. Most cars ford, chrysler, GM, to name a few release all the info through the obdII connector, but you need there software to read it. If a scan tool Co. doesn't pay for the info other than emissions failures it will not be in there software. We have factory scan tools for Detroits big three, and numerous others. You almost have to, if you want to do any programming. Even then when you have the equipment, to do updates to a cars computer you still have to log onto a manufactures website(for a fee) to do the update, info gets downloaded into the scan tool and then to the cars computer. As for doing additional testing, when you test with a voltmeter you normally will not find problems unless it is real bad, typically you need a lab scope and you would test the sensor under it's operating conditions. On a side note The shop I work at specializes in electrical testing and difficult problems(weather it is mechanical or electrical) on cars. We get alot of referances from other shops, and every now and then a new car dealer will send us one of theirs. Always makes you feel good when you fix one the dealer couldn't
Hope some of this info was useful, Eric