OBDII code reader..

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I have an OBDII (sp?) code reader from my old car. It's a scantool and support the CAN bus protocal also. I seem to remember this is what my 2002 996 uses. Is there any info that can be gleaned from doing a scan? Am I likely to interfere with anything? My car is running fine and not due a service for a while but I just want to be sure it's not telling me anything. Do all faults result in a check engine light?

If so where is the port to plug into?