OBDII lets you get access to some basic information about your engine like the coolant temperature or various sensor readings, but it's quite limited. For example, although your car might have an oil pressure sensor that doesn't mean you can get the data through the OBDII port.
Although most manufacturers also use the OBDII port to access more advanced features (like communicating with all the computers in the vehicle), the standard OBDII specification only deals with the engine computer, and even then it only gives you a subset of the entire range of data/features that are available.
You can also access error codes from the engine computer if it detects any faults. But even these aren't very useful unless you are a mechanic and understand the code. For example, let's say you retrieve a code that says "Oxygen Sensor Heater Circuit" which is a common code. So what can cause this code? Here's a few possibilities:
- Oxygen sensor itself is faulty.
- Fuse for the 12V supply to oxygen sensor heater blown.
- Power supply relay for the heater is faulty.
- Fuse for the relay coil is faulty.
- Drive circuit for the relay is faulty.
- Wiring from the 12V supply to oxygen sensor faulty.
- Wiring from the oxygen sensor to the computer faulty.
- Faulty computer.
Where do you start? Lots of people will simply replace the oxygen sensor. This will work for a lot of cases, but what if you replace the sensor and the same code comes back? Then you gotta start checking out everything else.
An experienced mechanic, who knows a lot about your particular vehicle, will have likely seen this problem and would know if the sensor is the best choice, or if there are other common problems (like a bad relay) which should be checked first.
Don't mean to rain on your parade, just trying to explain that knowing a code or having some data doesn't mean you know the problem. I get sick and tired of seeing all the ads for OBDII scanners (from stores like Pep Boys in the US or Canadian Tire up here) which imply that plugging it into your car will tell you everything that's wrong with it and save you money instead of going to a mechanic.