I am getting a little concerned with your eagerness to delve into the box but not knowing exactly what is wrong. Have you read the fault codes or taken the car to someone who can. I would lay money on the fault being the pressure regulation solenoid, but I would not go there unless that fault had been logged. The pressure regulation solenoid leads a very hard life in the AL4 and we have had to replace it twice on our Xantia.
It is not that expensive to replace or even get someone who knows what they are doing. I hope you are not driving the car around in limp mode, it is not designed as general drive mode and you may cause other damage. The difference in cost of a full overhaul and solenoid replacement is massive.