P0299 - Turbocharger/Supercharger "A" Underboost Condition
System
Powertrain (Engine & Gearbox)
Technical Description
The ECM detects that actual manifold boost pressure fails to reach target boost levels under load (typically >300-500 mbar under target for over 3-5 seconds), resulting in severe power deficit.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Boost Deviation (Target vs Actual) | Pressure (bar) | > 0.35 bar under target | Full throttle 2000-3500 RPM |
| Vacuum Supply to Turbo Actuator | Pressure (bar) | -0.65 to -0.85 bar | From vacuum pump |
| MAP Sensor Ambient Voltage | Voltage (V) | 1.60 - 2.00 V (~100 kPa atmospheric) | Key ON, engine off |
Full throttle 2000-3500 RPM
From vacuum pump
Key ON, engine off
Oscilloscope Waveform Analysis
500 ms/div, 1 V/divPossible Causes
Boost leak in charge air piping, split intercooler, or blown-off hose coupler
Torn diaphragm in wastegate/VNT vacuum actuator
Torn diverter valve / recirculating blow-off valve diaphragm
Worn turbocharger with excessive shaft play or eroded turbine wheels
Severely plugged DPF or catalytic converter creating extreme exhaust backpressure
Symptoms
3
Recommended Solution
Pressure-test the complete charge air system from turbo outlet to throttle body at 1.0-1.5 bar using smoke/air.
Check turbo actuator with vacuum pump to ensure it holds vacuum without bleed-off.
Remove and inspect the diverter valve diaphragm for tears.
Mechanic's Tip
On VAG 2.0 TFSI/TSI engines, a torn rubber diaphragm in the diverter valve (revision D/G) causes over 70% of all P0299 faults. Upgrade to the updated piston-style valve!
Common Combinations
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