Code 16990 on 1998 Volkswagen Cabrio

My Check Engine light is on and the car is throwing the 16990 code. Any suggestions for narrowing down the possible causes. Thanks!

Asked by for the 1998 Volkswagen Cabrio
Often cleaning the throttle body and setting adaption is all that's needed......
Thanks for the answers. Hopefully it will be warm enough to work on the car this weekend (I think I have the only house in CT without a garage!). I am praying it's just a dirty throttle body. I don't see any vacuum leaks, etc.
1 more answer , 2 more comments
That code is for the Engine Control Module (ECM) Is that the only code that is stored?
This is the code info: 17990 P1582 Idle Adaptation at Limit

The idle control system on your car compensates to keep the engine idling properly, when it has to over compensate, it sets a code. Vacuum leaks are a main cause, but it could also be the result of one of the sensors in the engine management system. Perform a visual check of the vacuum lines and intake hoses and look for loose clamps, disconnected hoses etc.
Sorry I wrote 16990; should have been 17990 (idle adaptation at limit) and that is the only code showing.