On my 2000 Volkswagen GTI VR6 the engine light was coming on and when diagnosing it with VAG-COM I saw code 16804 - Catalyst System; Bank 1: Efficiency Below Threshold.
The Cat and O2 sensors were replaced 5 months ago.
It turned out the problem was an exhaust leak at the rear of the Catalytic Converter. Tightening the clamp fixed the code and let me pass the readiness test. The catalytic converter is apparently very sensitive to back pressure.
[VAG-COM Basic Settings Block 46 (catalytic converter efficiency) improved from 0.71 to 0.46 (< 0.50 is passing). Block 34 (O2 sensor aging) improved from 1.75 to 1.91 (1.99 is new)]