What is a Suction Control Valve and why it could fail?

The suction control valve in the common rail fuel system basically acts as the throttle to the fuel pump, conserving energy in the system by allowing only the required amount of fuel to be pumped through the system. 

The suction control valve, like every other part in the later common rail diesel injection systems, operates at tight tolerances. Fuel contamination and wear inside the valve means that the ECU cannot command the correct fuel pressure to the injectors. Keeping in mind the incredible pressure (thousands of psi) inside the fuel rail, and that a faulty valve means that pressure is jumping around quite extremely, it makes sense that when diagnosing common rail diesel faults that involve drive-ability, this should be one of the first places the technician suspects.

Some people find their vehicle going into “limp mode”, and that is often due to the fuel pressure varying one way or the other past a limit set by the ECU.