Possible causes

– Air leakage after turbocharger
– Hoses between turbocharger and turbocharger control valve are loose, blocked or damage
– Hoses between turbocharger control valve and pressure servo are loose, blocked or damage
– Faulty turbocharger control valve
– Faulty boos pressure control valve pressure servo
– Sticking boost pressure control valve

Tech notes

First step is checking for air leak after turbocharger.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P1238 VOLVO Description

The code O1238 will set if RPM is above 3500 and engine is operating at a high load, control signal to turbocharger control valve is at maximum and boos pressure from boos pressure sensor is lower than approximately 2.9 psi under normal boost pressure (signal too low. If mass from mass air flow sensor is high than desired value (signal too high) it is interpreted as a fault by the Engine Control Module (ECM). When the code is set, a reduce boos pressure is used as a substitute value.