First things first, has it work correctly before and for how long?
Secondly, have you worked on or played with anything in your vehicle?
Assuming it has worked correctly for a period of time and you haven't touched anything, I'd recommend disconnecting the power wires to the P1900 for a few hours and then plugging it back them back in. That will reset the unit.
I've had a P1900 and a P2140 and they're very glitch to say the least.