If the high pressure pump works well, but your Car still can’t start, let’s move to next step to check the common rail pipe.
On the common rail, there are one rail pressure sensor, and also one Pressure limiting valve.
The common rail pressure sensor is located on the fuel rail. Its function is to monitor the fuel pressure in the common rail.
Sensor will send signal to ECU to tell if the pressure in the common rail is higher or lower than standard. When ECU receive the signal, it will control the fuel metering valve( Actuator) to supply less and more fuel into the common rail. Thus the pressure in the common rail will be control by ECU into a normal range.
Fig. 1 shows a typical common rail pressure sensor.
common rail pressure sensor
Pressure limiting valve is  to protect a common rail fuel injection system from overpressure.
If the the pressure in the rail is too high, it will push the limiting valve, make it open, so that the fuel will flow into the fuel tank from the pressure limiting valve and oil-reture pipe.
Normally, there won’t be fuel in the oil ruture pipe, because the pressure is in the common rail is been controlled well. So when  checking the the engine, you can disassemble the scew on the oil reture pipe, to see there there is fuel spraying. That means the pressure in the common rail is lower than stardard to start the car.  The common condition is Pedal to the bottom, the maximum speed is 1500rpm.
 common rail pressure limiting valve
Fig. 2 shows a typical common rail pressure limiting valve



Here are some common rails for your reference:


More Article:

3 tips for checking the fuel metering valve

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