Replacing the fuel pressure sensor was not a waste of money. They wear out, engine runs rough and/or crank over no start. The fuel tanks needs to be dropped for PEM and/or fuel pump replacement.
It would be a good idea to replace both PEM and pump at the same time. Try to run the tank below 1/4 or more. It is very difficult trying to handle a full tank of gas even using the Volvo jack and tank arm supports. Once they drop the tank, they may find a mouse nest and/or wire chewing.
Will the new PEM and tank pump fix it, it is hard to say.
If your car is metal to metal on the brakes, I can guarantee new pads/rotors will stop the squealing noise
when pressing down on the brake pedal. This other stuff, I stopped saying, that will fix it years ago.
Below is VIDA info about the code and ECM functions.
Start on page 4 of 52 for PEM and fuel pump info in ECM PDF.
Dealer or non dealer repair center doing the repairs.
Dealer, you have lifetime warranty on parts/labor as long as you own the car in your name.
284: ECM-P008900 Fuel Pressure Regulator Performance. General Failure Information. No sub type information, B5254T5
Diagnostic trouble code (DTC) information ECM-P008900
Condition
The engine control module (ECM) controls the fuel pressure via the fuel pump (FP) control module using a pulse width modulation (PWM) signal. The fuel pump (FP) control module then controls the fuel pump (FP) depending on the pressure requested by the engine control module (ECM). The diagnostic trouble code (DTC) is stored if the control capacity of the engine control module (ECM) for fuel pressure reaches the highest or lowest value and the desired fuel pressure (target value) is not achieved.
The diagnostic trouble code (DTC) can be diagnosed when the engine is idling.
For information about fuel pressure regulation, see VIDA Design and Function.
Substitute value
None.
Possible source
Leakage in the fuel lines
Damaged fuel pressure sensor
Damaged fuel pump
Damaged fuel pump control module.
Fault symptom
Malfunction indicator lamp (MIL) lit
Deterioration in performance
The engine stops.
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Fuel pressure regulation, diagnostics
General
The fuel pump control module has no functions for diagnostics. Fuel pressure control components and functions are diagnosed by the engine control module (ECM).
The following components and functions are diagnosed by the engine control module (ECM). Diagnostic trouble codes (DTCs) can be stored for each component and function:
Fuel pump control module, a diagnostic checks the power supply. A diagnostic trouble code (DTC) will be stored and the fuel pump will not work if there is no power supply to the fuel pump control module
Fuel pressure sensor with temperature sensor, a diagnostic checks the pressure and temperature signals to see if they are outside the normal operating range of the sensor. If a fault in the fuel pressure sensor is detected, the fuel pump (FP) will run at a full power and the pressure is governed by the by-pass valve in the fuel tank
Fuel pressure, the engine control module (ECM) compares the requested pressure with the measured pressure(signal from the fuel pressure sensor). If the measured pressure deviates excessively from the requested pressure, or if the pressure pulses, a diagnostic trouble code (DTC) will be stored for incorrect fuel pressure. The diagnostic is not active if a diagnostic trouble code (DTC) for the fuel pressure sensor is stored. A diagnostic trouble code (DTC) for fuel pressure will be stored if there is a fault in the fuel pump
Communication cable. The engine cannot be started if there is a fault in the communication cable between the engine control module (ECM) and the fuel pump control module. A diagnostic trouble code (DTC) will be stored for the fault.
For more information about diagnostic trouble codes (DTCs) see Information, Diagnostic trouble codes (DTCs) in the diagnostic tool.
Components and functions which cannot be diagnosed:
If there is a fault in the by-pass valve in the tank unit, the starting process of the engine will take longer if the valve is leaking fuel when the fuel pump is off. If the valve does not open when there are pressure peaks in the fuel system, engine operation will be affected negatively, especially after engine braking.