1996 Nissan Altima GXE 4 Cyl 2.40L
The engine speed increase itself ( sometime )
we are dificult to find the root cause of the incident, It sometime when engine idle the engine speed is increase the acceleration itself, what is happend on this incident ? The code P0600.
The P0600 is a transmission fault code that I do not think has any relation to your idle speed problem.The code is titled - TCM Communication Line Circuit Malfunction - With Automatic Transmission. TCM is Transmission Control Module. I can tell you that a vacuum leak can cause the idle speed problem on your Altima and the most likely source to check would be the intake manifold gaskets as they are very common to fail.