Q: engine code 234 on 2001 Volvo S40

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engine won't idle
(2) Answers
Hello, P0234 is for a boost signal to the turbocharger, indicating there is an issue with the air coming into the engine, or at the turbo itself. You could have excessive air coming into the engine possibly from an air tube leak or maybe a vacuum leak of some sort. You should refer to a local shop to be able to help you on this one. Good luck.
Just to tag onto that, if the wastegate or diverter valve/blow-off valve is not opening, you will have overboost.
If the wastegate is internal, the vacuum actuator could have failed, which would not allow the wastegate linkage to push or pull the wastegate.
This is particularly dangerous due to higher than normal combustion chamber temperatures, extreme lean running conditions, and heat at the turbocharger and exhaust manifold.
If your vehicle feels like it has more power than normal, or you are seeing grey-blue smoke from the exhaust, the vehicle should be towed, not driven.