p1494 code

1998 jeep grand cherokee check engine code p1494.
Says it is a leak detection pump malfuntion.
Anybody had this problem or know how I can trouble shoot?
what is and where is the leak detection pump?

any help would be appreciated.

Thanks.
vin

It's part of the evaporative emission system. The LD pump is under hood, drivers side, next to the Master cylinder.
Full description for the code:
Incorrect input state detected for the Leak Detection Pump (LDP) pressure switch.
Most likely not the problem, but check your gas cap.

ok thanks I'll check it out.
Let you know what I find.