I have a 1999 STS and the "Service Engine Soon" light is on. When i pulled the code it came up as .. Code P1646.. Evaporative emission (EVAP) vent Solenoid Control Circuit.

Does anyone have a clue as to what this is? And costs to repair?

The EVAP system takes evaporative gas fumes from the tank and burns them. The EVAP control valve and solenoid are on the rear cam cover I believe on a '99. It can be identified by an electrical plug on the solenoid and I think there are two vacuum lines to the valve. Cost should not be terribly high.