P0320 98GT

[phoenix99]

Hi, since yesterday my 98GT began to "knock" when I drive.
Today, the check engine lite and with my xcal2 it's give me the error code P0320.
P0320 Ignition Engine Speed (PIP) Input circuit fault.

What can I do to correct this?
What cause this?
My car run very fine 3 days ago and now I can't get out of the driveway ...

Need help please !

thanks !