1998 Hyundai Tiburon cel 1124, lean fuel

I just recently got the engine light on the car come on.
Scanner indicated P1124. ( Fuel trim error-System Lean)
This just happened all of a sudden.

Now the car runs fine. At high speed, no problems, when the car is say idlling,
or stuck in stop and go traffic, the light comes on again.
The rpm needle is at a constant speed, it doesn't go up and down.
When in the car, you can feel that the idling is not smooth, it's slightly rough, very hard to notice.
Out of the rear tailpipe you can hear the engine misfiring or some sort, or rough idling.

Is the p1124, the actual sensor that's bad, or it could be other sensors that might be bad, and triggering the p1124 code?
and where is the sensor located for the p1124 code?

Some causes listed where:
low fuel pressure
failed fuel pump
Vaccuum leak

Could it be any of those listed above?



I'm not even sure where to start, any ideas?