In some high-mileage Renaults, the turbocharger bearings wear out, causing shaft play. This leads to inconsistent boost pressure. However, P158B from purely mechanical turbo failure is less common than VGT carbon buildup.
If actual boost consistently stays below requested under load → Underboost issue (vacuum leak, stuck vanes, boost leak). If actual boost overshoots requested → Overboost issue (stuck vanes in low position, faulty solenoid). p158b renault
Carbon buildup can physically block the idle actuator from moving as intended. Cleaning the intake and throttle plate with a specialized cleaner can sometimes clear the fault. In some high-mileage Renaults