I have 3 Saabs, and this is my first Volvo. I checked the S40 forum, and no help on this code, any ideas, fuel mileage is lower than usual. I replaced the O2 sensor 1 year ago, and a new thermostat (another code). No this P0234 code for Over boost? Other than this the car is driveable?
I have Emission inspection coming up, need to get this fixed, I reset the OBD2, and it the code came back after a day or so (not immediate)
Any Help??

 

I'd play around with this piece to see if it's not opening the wastegate properly: (BCS) Boost Control Solenoid

I'd also check the wastegate itself to make sure it moves as it should. Avoid adjusting the wastegate without the proper tools.

 

Is it okay to lubricate the wastegate?
How can you check if the solenoid is working properly?

 

Go sparingly on lube as those parts get very hot. Stuff like WD-40 is flammable. I was thinking disconnect the wastegate and play with it by hand to make sure it moves freely. You can also put air pressure to the line and watch the wastegate open and close. The 'proper tools' I referred to above is an air gauge teed into an air line where you control the pressure. This allows you to see at exactly which psi the wastegate opens.

For the BCS I'd do the pressure test also. You may need the engine running so I'd set it up cold before starting the engine. With a gauge for accuracy just keep adding pressure to see at what point the BCS allows pressure to the wastegate actuator. Check the above link at IPD. They have a youtube video that I can't access here in China. Hopefully it explains more. They also sell a kit with a gauge to check the wastegate actuator although you can put something together yourself for less money.

If all is well then I'd suspect the front MAP sensor (there are 2 total). I believe this is the actual part measuring your overboost. If it's faulty then that could explain the lower fuel economy.