The reason for the warning light could be a number of things.
I.E DPF - needs regenerating
2.EGR valve
3.Lambda Sensor fault.
4.Loose connection
5.ECT ( engine coolant temp sensor)

You could try and do the regenerating yourself. You need to take the car onto a motorway/Dual carriage way of enough length.
Then run the car so that the rev counter is at a min if 2000rpm for approx 30 mins.

This can in some cases allow the build up to be burnt of (from the PDF)
Other than that you may need to have a diagnostic check carried out which should then show a fault code(ie P ---- 4 numbers) which can then be related to the fault.