Sadly there are numerous reasons for the warning light to appear on the Dci engines - even running out of fuel will trigger the darned thing. It could also be water in the fuel filter or a duff EGR valve or faulty glow-plugs. I know that gives you loads of options but it may be prudent to have a close look at the under bonet connections first just in case something has come adrift
Exhaust Gas Recirculation
EGR is a NOx (nitrogen oxide and nitrogen dioxide) reduction technique used in most gasoline and diesel engines.
EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. Intermixing the incoming air with recirculated exhaust gas dilutes the mix with inert gas, lowering the adiabatic flame temperature and (in diesel engines) reducing the amount of excess oxygen. The exhaust gas also increases the specific heat capacity of the mix lowering the peak combustion temperature. Because NOx formation progresses much faster at high temperatures, EGR serves to limit the generation of NOx. NOx is primarily formed when a mix of nitrogen and oxygen is subjected to high temperatures.