I have recently had the same problem, and spent entirely too much money trying to figure it out. There is NO accurate way to determine exactly what it its.
It can be:
Vacuum leak in the engine
bad O2 sensors
absolutely no reason at all
You can clean your O2 sensors; my suggestion would be to use carb/intake cleaner, however, it's not likely to make any changes.
If you do elect to replace an O2 sensor, bank 1 is the upstream (after manifold) O2 sensor.
In my situation, it was a bad catalytic converter. The catalyst inside had literally turned sideways, and the code is thrown from the readings between the two sensors being too similar.
2001 Golf GL 2.0L 5-speed
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Monster truckin' at 26.5" Front and 27" Rear.