Diagnostic Overview
Volvo fault code P0299 usually indicates turbocharger underboost or low boost pressure. The ECM is commanding more boost than the engine is actually producing. This can cause low power, excessive smoke, poor fuel economy, slow throttle response, and possible derate depending on severity.
Likely Causes
Charge-air cooler leak; loose intake hose; boost leak; exhaust leak before turbo; sticking VGT actuator; failed turbo actuator; restricted air filter; faulty boost pressure sensor; worn turbo.
What To Check First
Pressure-test the charge-air system, inspect intake boots and clamps, check exhaust leaks, compare boost sensor data, and test turbo actuator movement.