VW Passat Emissions Control Light

[Treacletown]

This has on permanently when I was driving on a dual carriageway at 55 mph and 2000 revs. Any ideas what might have caused it?

Thanks In advance

[Sardonicus]

Get it plugged in to see any stored DTC's and the live data studied be a failed 02 sensor , un-metered air leak, wiring failure, misfire etc etc etc etc you get the drift