I believe this is related to the solenoid that controls the variable veins. Had the same issue P0236 (Underboost) after doing major front end work on my 2.4. Not sure if the 1.9 has the same layout......
Turned out to be a 50 Ohm resistance across the multi electrical connector on the front right hand side. You need to check resistance of wiring back to ECU, should be 0 Ohms and remedy where required. I ended up having to bridge the multi connector as no end of switch cleaner would do the trick and it's impossible to get a small file in there.
If resistance is OK then it's probably the solenoid. On the 2.4 it's bolted above the left hand side of the rad and has 2 small bore rubber pipes coming from it. Easy job to replace.
If you chase the answer, it moves away.