How do you test the primary fuel pump on a Peugeot 406 HDI diesel engine?

To test the primary fuel pump on a Peugeot 406 HDI diesel engine, first ensure the ignition is turned off and disconnect the fuel supply line from the pump. Then, turn the ignition on (without starting the engine) to activate the pump and check for fuel flow from the pump outlet. If fuel is flowing, verify the pressure using a pressure gauge; it should meet the specified range for the engine. If there's no flow or inadequate pressure, the pump may need to be replaced.