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If you have Low Fuel Pressure at the Rail then you either have an issue in the supply of fuel to the Rail via the High Pressure Pump or you have Injectors that are leaking past and not holding pressure at the rail.
You have stated that you have replaced the filter. I would suggest you need to carry out a Fuel Pressure Test. I would initially test the Output from the High Pressure Pump. If that is ok then this eliminates the Fuel Supply to the Pump and also the Fuel Supply to the Rail. This would point to Injectors leaking past so the next stage would be to do an Injector Leak Test. This will establish if the Injectors are leaking past.
If this all test out ok it could be that the Rail Pressure Sensor is faulty and so the wiring to this should be checked and the Sensor replaced as necessary.