drain water separator stays on
Sorry, I am not familiar with diesels. I am sure others here are and can help. but I found this that might…
The “drain water separator” warning light on your 2008 Ford F250 is an indication that there is water in the fuel system. The fuel conditioning module needs to be drained and the fuel system serviced. Once the fuel conditioning module is drained and the fuel system service is completed, make sure the fuel/water separator drain valve is completely closed to prevent air from entering into the fuel system. Open the fuel/water separator drain valve and drain the fuel into a suitable container. Disconnect the Fuel Pump (FP) and the water in fuel sensor electrical connectors. Disconnect the fuel tank-to-fuel conditioning module fuel supply tube and fuel conditioning module-to-fuel tank fuel return tube quick connect coupling. Disconnect the fuel tank-to-fuel conditioning module-to- engine fuel supply tube and engine-to-fuel conditioning module fuel return tube spring lock coupling. Remove the 3 mounting nuts and the fuel conditioning module. To install, tighten to 20 Nm (177 lb-in).1
If you are not comfortable performing these steps yourself, it is recommended that you take your vehicle to a certified mechanic for service.
I hope this helps!