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Turbo Under-Boost

Turbo Under-Boost Brand: VW, Audi, Seat, Skoda Models: All vehicles with variable vane turbochargers


Customer complaint is that vehicle lacks power or does not attain top speed.

Fault Code

Fault code P0299 = Turbo under-boost


  • Connect AssistPlus and check for any engine or engine management faults, especially any fault code indicating turbo under-boost.
  • If a fault code is present, connect a mityvac to the turbo waste gate, pull a vacuum and see if it holds.
  • If vacuum does not hold, the diaphragm has failed. Note that in most common-rail diesels, this diaphragm can be replaced separately.
  • If vacuum holds, then leave vacuum on and drive the vehicle in this state. Does the vehicle drive OK?
  • If the vehicle now drives OK, this indicates that all the turbocharger mechanicals are OK and the fault lies with the turbo control.
  • Inspect the vacuum pipes and hoses for deterioration or disconnection to and from the control solenoid (N75). Check for the correct operation of the N75 valve. 
  • If the fault is still present, this indicates a turbo mechanical problem because turbo control has been eliminated as a cause of the problem. There could be a problem with either:
    • The turbo itself
    • Blocked exhaust
    • Blocked or leaking intake
  • To rule out a blocked or leaking intake:
    • If you have a common rail engine, contact our Fault-finding Team to check the system under boost.
    • If you have a PD diesel engine and wish to test the intake boost system, you can do so with the engine on idle. Use your AssistPlus device:
  • This will boost the intake in steps so that you can check the system without driving the vehicle.
    • You can also check the boost live data in measuring groups. This shows you the actual and specified boost pressure. Driving under load, the values should match, as you only get boost while driving.
  • A blocked exhaust normally shows up in conjunction with a DPF fault or a warning light. Rule this out by reading the back-pressure and/or venting the exhaust system.
  • If the checks carried out have allowed you to rule out the above, then the turbo itself will need to be checked for internal damage. Repair or replace as required