Cars since 1996 have the OBD-II On Board Diagnostic system where an OBD-II Scanner can pull codes and get data from the computer.
Systems can be monitored all of the time by the computer, but the OBD-II scanner available for under $100 from Sears or Walmart can help to get codes and descriptions of codes if your car has a Service Engine Soon light on.
Yes, it's always active. A light will come on when it detects a fault. You can also use a good scanner to interrogate the system, actuate valves, etc.
Please see my tip at http://www.fixya.com/cars/r5932051-check_engine_light_check and let me know if you have questions.
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