What does it mean?
SPECIAL NOTES: Carmakers manage turbo/supercharger boost pressure in many and diverse ways- from simple vacuum operated actuators that dump excess pressure through a dump valve (aka waste gate), into the exhaust system, to highly sophisticated control modules that regulate the turbo chargers’ spin rate to control and manage boost pressure.
Where simple vacuum operated boost control systems (dump valves) are used, many generic (P0XXX) OBD II codes, such as P0234, P0235 P0236, P0237, P0238, P0239, P0240, P0241, P0242, P0243, P0244, P0245, P0046, P0247, P0248, and P0250 all describe various issues and problems of the (vacuum) boost control system on normal non-variable turbos, but be aware that these generic codes should NOT be confused with OBD II code P00AF, which refers to a specific type of variable boost control system that uses various methods to control/regulate the amount of exhaust gas that is allowed to enter the turbo at any given engine speed and set of operating conditions.
In light of the above, non-professional and DIY mechanics are strongly urged NOT to attempt any diagnostic/repair procedure on any turbo/supercharger boost control system without first reading the section in the manual on turbo/supercharger boost control as it applies to the application being worked on. Moreover, non-professional mechanics are urged to gain a full understanding of how the boost control system works, before attempting a diagnosis and/or repair of OBD II code P00AF.
END OF SPECIAL NOTES. OBD II fault code P00AF is a generic code that some manufacturers define as “Turbocharger/Supercharger Boost Control “A” Module Performance”, and on applications that use this code and definition, the code sets when the PCM (Powertrain Control Module) detects a discrepancy between actual boost pressure, and the desired, or target boost pressure. Note that OBD II code P00AF refers specifically to VGT (Variable Geometry Turbos), also known as VNT (Variable Nozzle Turbos).
Also, note that VGT/VNT turbo chargers do not use waste gates or dump valves to avoid turbine over speed situations, or to dump excess pressure. In terms of operation, variable turbo chargers employ a system of moveable vanes to both direct the flow of exhaust gas to strike the vanes of the turbine wheel at the optimal angle, and on some applications, to regulate the amount of exhaust gas that strikes the turbine wheel.
Thus, if the vanes are adjusted so that the spaces between them decrease, the exhaust gas passing through the vanes is sped up, which in turn increases the turbine wheel’s speed, thereby increasing the boost pressure. Conversely, if the spaces between the vanes are increased, the gas speed decreases (due the larger volume it flows through), which allows the turbine wheel’s speed to decrease, thereby decreasing the boost pressure.
The advantages of this system lay in the fact that all of the exhaust gas can be employed to drive the turbo, which reduces turbo lag and exhaust emissions while at the same time, matching the turbine speed to the engines’ boost requirements through a wider range of engine speeds than is possible to do with a fixed-vane turbo charger.
It is perhaps worth noting that non -variable turbochargers are 100% efficient only when the engine is running at a constant speed, which is the only time when the turbine wheel also rotates at a constant speed. In practice, the moveable vanes in variable turbochargers are controlled by a dedicated control module through either a vacuum-operated actuator, or a DC stepper motor.
This arrangement makes it possible always to match, or nearly match, the turbine speed to the engines’ boost requirements, which has the happy consequence of largely eliminating over boost and turbine over speeding conditions, without the need to employ a separate device to control boost pressure by dumping excess pressure into the exhaust. As a general rule, variable turbo chargers work best when the vanes are about 60% open, but note th