OBD II Diagnostic Software

The process of controlling the MIL on an OBD I system was quite simple. If something major broke, the light came on as soon as the problem was detected and stayed on until the fault was fixed. If an intermittent fault "fixed" itself the light would go out automatically when the key was cycled off, then on again.

OBD II diagnostics are more complex. Every time the vehicle is driven, numerous system diagnostic procedures run. Some tests happen once per trip and some run continuously. Most tests will only run when specific operating requirements such as a minimum coolant temperature, RPM, vehicle speed, etc. have been met. In addition, most faults must occur more than once before the MIL will be illuminated, but a serious misfire must turn the light on immediately.

Keeping track of all these factors is a complicated task, it undoubtedly takes more memory than we had in our entire fuel system a few years ago just to decide when, and in what order, to run the diagnostics.

The diagnostic software in an OBD II system must:

^ Turn the MIL on and off.
^ Record and clear DTCs.
^ Capture freeze frame data for the FIRST emissions-related DTC recorded (data for a severe problem like misfire will replace data stored for a less serious fault).
^ Remember the operating conditions present the first time certain faults occur so that following tests can be run under similar conditions.
^ Tell you if each individual diagnostic test has had the opportunity to run since the codes were last cleared.