Stalled Tachometer

[LeinadStrebor]

I have a 2004 206 Air Nautique with just over 800 hours on it. Not sure how long its been going on, but the tachometer has stopped working. Regardless of whether of the boat is idling under power or completely cut off, the tachometer reads at about 900 rpms and never moves. Any ideas why it might do this or what can be done to fix it?

Thanks