OER Series Fuel Return Line Assembly 2011-2016 GM 6.6L Duramax LML/LGH XDP Xtreme Diesel PerformanceRestore the fuel system in your 2011 2016 GM 6. 6L Duramax LML LGH with the XDP OER Series Fuel Return Line Assembly XD607. If you have dealt with a CP4 pump failure, debris and contamination have most likely worked their way throughout your fuel system. When these metal shavings and other contaminants gets in the return line it can block the regulator and prevents adequate return fuel pressure. In the event of a failed CP4, it is recommended that you