Understanding the system and symptoms
Reduced boost can arise from charge-air leakage, wastegate control faults, diverter valves, sensors, exhaust restrictions or damaged turbocharger hardware. The turbocharger itself is only one candidate.
How a technician narrows down the cause
Compare specified and actual boost under controlled conditions, then pressure-test charge-air plumbing and inspect clamps and intercooler connections. Verify actuator movement and command with the correct engine-specific procedure. Check pressure sensor plausibility before attributing underboost to a turbocharger.
Model-specific context: Audi S5; researched applications 2018; platform S5 B9 Sportback. Exact applicability should be checked against the vehicle identification number and equipment.
What owners may notice in researched cases
- MIL/lack of power with recurring P0299 underboost.
Diagnostic observations from the source cases
The available technical cases describe checks such as Connect wastegate directly to vacuum source; N75 resistance, voltage and continuity; Smoke and pressure tests; Remove and inspect main vacuum hose. These checks help distinguish competing causes before recommending replacement.
Possible causes identified in individual cases
The underlying causes documented in individual cases included Internally swollen and blocked main vacuum hose from vacuum pump to N75. These are examples of verified findings in those cases, not a prediction of what has failed on every Audi S5.
Repairs reported in the source cases
Repairs reported to resolve documented cases included Replace main vacuum hose/three-valve assembly. A repair that succeeded on one vehicle should only be recommended for another after confirming the same fault.
Why confirmation matters
Why diagnosis matters: The research also recorded unsuccessful or temporary repair attempts, including Replace N75 and diverter valve — no change; Replace N75 again — no change. This illustrates why replacing a commonly suspected part without testing can waste time and money.
Fault codes recorded in the research
Codes mentioned in the research: P0299. Codes can have different interpretations depending on model and conditions.
Verification and next steps
Confirm the requested boost is achieved without overboost or underboost faults, and inspect for oil or exhaust leaks after any turbo-related repair.
If your Audi S5 is showing these symptoms, Mich’s Foreign Cars in Hickory, NC can discuss an appropriate diagnostic appointment. Call 828-324-6012 and provide the year, engine, warning messages and any available fault codes.