P00AF – Turbocharger/Supercharger Boost Control “A” Module Performance

By Reinier (Contact Me)
Last Updated 2016-11-17
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P00AF Turbocharger/Supercharger Boost Control "A" Module Performance
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Table of Contents

  1. What Does Code P00AF Mean?
  2. What are the common causes of code P00AF?
  3. What are the symptoms of code P00AF?
  4. How do you troubleshoot code P00AF?
  5. Codes Related to P00AF
  6. Get Help with P00AF

What Does Code P00AF 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 that this is a “ball-park” value that varies between applications, although the differences usually amount to only a few percent. Nevertheless, always consult the manual for the application being worked on to determine the optimum vane setting for the turbo charger on that particular application. Regardless of the application though, turbocharger efficiency drops off rather sharply when the vane opening is changed either way (increased or reduced), from the mid-opening position, but despite this, the advantages of variable geometry turbochargers far outweigh any disadvantages.

In a fully functional system, the adjustments to the turbo charger vanes are seamless due to the constant communication between the PCM, turbocharger control module, and the turbo turbine speed or pressure sensor. However, when the PCM detects a discrepancy between the actual, and the desired or target boost pressure as the result of a malfunction of any part of the turbocharger boost pressure-control system, it will set code P00AF, and illuminate a warning light.

The image below shows the basic construction of a variable turbocharger. Note that the example shown here is only one of several possible designs, but irrespective of the actual design, all variable geometry turbo chargers achieve the same result-, which is to improve engine performance, while reducing exhaust emissions.


What are the common causes of code P00AF?

Some typical causes of code P00AF could include the following-

  • Burnt, damaged, shorted, disconnected, or corroded wiring and/or connectors
  • Vacuum leaks caused by damaged, split, dislodged, hardened, or perforated vacuum lines
  • Insufficient vacuum due to a defective vacuum pump
  • Defective vacuum actuator
  • Defective vacuum control solenoid
  • Defective stepper motor
  • Mechanical defects of the variable vanes
  • Mechanical failure of the turbocharger itself
  • Defective, or malfunctioning boost pressure or turbine speed sensor
  • Clogged catalytic converter
  • Clogged or defective DPF (Diesel Particulate Filter). Note that on some applications, code P00AF can be set by a low exhaust reductant fluid level, or even a miscommunication between the SCR (Selective Catalytic Reduction) injection and turbo boost control modules. Note however that in these cases, there will likely be specific SCR related codes present along with P00AF.


Note that this list of possible symptoms is not exhaustive, and there may be make-and-model specific causes of this code that may, or may not affect other applications. Always consult the manual for the application being worked on for detailed information on the most likely causes of code P00AF on that particular application.

What are the symptoms of code P00AF?

Typical symptoms of code P00AF are much the same across all applications, although the severity of one or more symptoms may vary between applications. Below are some typical symptoms associated with this code-

  • Excessive turbo lag
  • Excessive oil consumption
  • Loss of power that can vary from slight to severe
  • White smoke from the tailpipe may be present
  • Mechanical, or whistling noises may be present at some engine speeds
  • In some cases, the turbocharger may suffer severe, to fatal mechanical damage

How do you troubleshoot code P00AF?

SPECIAL NOTES: Non-professional mechanics should take note that turbochargers (and superchargers for that matter), do NOT contain any user serviceable parts. If it is suspected that the cause of code P00AF is a failed turbocharger, the best course of action would be to refer the vehicle to specialist repair shop that deals with turbocharger repair. Turbocharger repair is a highly technical and specialized field and turbocharger repair should only be undertaken by, or entrusted to, repair facilities with proven experience in repairs of this type.

Also, be aware that reprogramming a PCM with performance specs can cause the failure of variable geometry turbochargers, and especially if the reflash involves changing DPF (Diesel Particulate Filters) self-cleaning intervals or settings. Turbochargers of this type depend on the free flow of the exhaust stream to work to best effect, and should anything, such as a clogged DPF or catalytic converter inhibit the free flow of the gas stream, the turbo can suffer damage due to overheating. END OF SPECIAL NOTES.

NOTE #1: Owners and drivers of 2013 VW Golf, Jetta, Beetle, Jetta Sportwagon, Golf Wagon, or Passat vehicles fitted with 2.0L turbocharged engines are advised NOT to replace any parts on the application should code P00AF – “Turbocharger/Supercharger Boost Control “A” Module Performance” present itself on these applications. Instead, owners should contact their nearest dealer, or the Volkswagen Technical Helpline for detailed diagnostic information to avoid replacing parts and components that will likely not resolve the issue.

NOTE #2: Due to the wide application of variable turbochargers on anything from small, 900cc diesel engines, all the way to the premium 3.0L diesel segment and beyond, as well as on a host of gasoline engines, it is not possible to provide detailed diagnostic/repair information for code P00AF on all applications. However, the “generic” procedure(s) outlined below should enable most non-professional mechanics to diagnose most, if not all, problems in the control circuit of non-variable turbochargers.

NOTE #3: Apart from a repair manual that includes a color-coded wiring diagram, and a high quality digital multimeter, diagnosing code P00AF may require a hand-held vacuum pump fitted with a graduated gauge.

WARNING: The information provided here is intended for general informational purposes only, and as such, it should NOT be used in any diagnostic and/or repair procedure without proper reference to the relevant section in the manual for the application being worked on. Failing to refer to the manual could result in a misdiagnosis, damage to the application, and the almost certain unnecessary replacement of parts/components.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

NOTE: If any other codes are present along with P00AF, resolve these codes in the order in which they were stored before attempting a diagnosis/repair of P00AF. Certain codes, such as those related to catalytic converters, DPF’s, (Diesel Particulate Filters), and engine vacuum system leaks have the potential to set code P00AF.

Step 2

Knowing how variable boost control systems work makes diagnosing faults in the system considerably easier, so spend an hour or so reading the relevant section(s) in the repair manual, and be sure to use the manual to locate all the parts and components that make up the system. Do not proceed to Step 3 unless you have a full understanding of how the system works, where all the components that make up the system are located, and how these components interact to make the system work as intended.

Step 3

The variable boost control system will either be vacuum -, or electrically operated, so once you have identified the various components, proceed as follows-

If the system is electrically operated with a stepper motor and the scanner has control functions, command the system to the fully open position, and from there, to the fully closed position. The degree of opening of the variable vanes will be displayed as a percentage, but physically checking if the vanes actually move though their whole range of movement is problematic, since the mechanism is located inside the turbocharger.

Nonetheless, the presence of code P00AF is a good indication that the vanes either do not move through their whole range of movement, or that they do, but that their actual position does not agree with their percentage of opening displayed on the scanner. Determining which is which is not easy, but in most cases, it is safe to assume that the problem involves a discrepancy between the vanes’ actual and their displayed positions.

One way to investigate this is to disconnect the linkage between the stepper motor, and the turbocharger, and to move the vanes’ actuating lever manually. The movement should be smooth and without significant resistance. Any stickiness, catching, or momentary high resistance to movement would indicate a problem with the range of movement, but to confirm this, use the scanner to command the stepper motor to move from the fully closed, to the fully open position multiple times.

The motors’ movement should also be smooth and without any “jerkiness”, or interruptions. If it is not, the motor is either defective, or there is a problem in the wiring and/or sensors that control the motor.

Step 4

If the stepper motor does not rotate freely, or does not rotate at all, consult the manual to determine the correct procedure to test the resistance, and continuity of the motor, as well as all associated wiring, sensors, and connectors. Be sure however to disconnect the boost control system from the PCM and other relevant controllers during this step to prevent damage to the controller(s).

Compare all obtained readings with the values stated in the manual, and make repairs, or replace components as required to ensure that all electrical values fall within the manufacturer’s specifications.

NOTE: If no damage to wiring/connectors is found, the stepper motor works, the variable vane mechanism is not sticking, the turbocharger itself is known not to have failed, and all electrical values fall within specifications, suspect a defect in the boost control module, or in rare cases, a defect in the PCM. Be aware though that both controllers might require reprogramming if they are replaced.

Step 5

If the variable boost control system is vacuum controlled, locate all associated vacuum hoses, actuators, and control solenoids. Inspect all vacuum hoses for visible damage such as splitting, cracking, hardening, or perforations, and replace all hoses and lines as required.

Step 6

If no damage to vacuum lines is found, locate the vacuum actuator that controls the variable vanes, and connect the vacuum pump to it where the vacuum system normally attaches to the actuator.

Consult the manual to determine the minimum vacuum (measured in inches of Mercury) that will cause the actuator to move, and draw that amount of vacuum with the vacuum pump. If the actuator diaphragm is not perforated and the test equipment is not defective, the vacuum will hold steady at that level. If the vacuum decays, however slowly, the vacuum actuator is defective, and must be replaced.

Most vacuum systems incorporate a position sensor that supplies the PCM with a signal voltage that changes as the vacuum increases/decreases. Consult the manual on the correct procedure to test the position sensors’ resistance and continuity to either confirm, or eliminate the position sensor as the cause of the code. Compare all obtained readings with the values stated in the manual, and replace the sensor if discrepancies between measured and specified values are found.

However, if the position sensor is found to be in working order, consult the manual on the value for the vacuum required to open the variable vanes fully, and apply that vacuum to the actuator, but be aware that exceeding this value proves nothing, since the engine can only supply the specified vacuum.

Be sure to keep the scanner connected during the vacuum test, since it will display the changes in the signal voltage sent to the PCM as the vacuum increases. Consult the manual to determine the value for the signal voltage when the variable vanes are fully open, and verify that this value is reached when the maximum allowable vacuum is applied to the actuator. When this value is indicated on the scanner, it should not be possible to move the actuating mechanism any further by hand- if however, this is found to be possible, suspect a maladjusted actuator, a defect in the variable vane mechanism, a defective position sensor, or in rare cases, a failed PCM, or boost control module.

NOTE #1: Be sure to disconnect the vacuum actuator from the variable vane mechanism to check manually that the mechanism moves freely throughout is full range of movement. Any stickiness or excessive resistance to movement would indicate a defect in the mechanism that will require professional diagnosis and repair.

NOTE #2: Before you start replacing components, consult the manual on the correct procedure to test the vacuum control solenoid, which supplies/controls the vacuum to the vane actuator. Replace the vacuum control solenoid if it is found not to conform to the manufacturer’s specifications.

NOTE #3: Be aware that on diesel applications, the vacuum required to operate the variable vanes on the turbocharger is not supplied by the engine, as it is on most gasoline applications, but by either a dedicated vacuum pump, or the vacuum pump that supplies the vacuum for the brakes. It is therefore imperative to test the quality of the vacuum created by the pump, as well as to the check the correct operation of all control solenoids, check valves, connectors, and other associated components to verify that the correct vacuum is available to the turbocharger at all times. Consult the manual on the correct procedure to test these components, and replace any that do not conform to specifications.

Step 7

The generic steps outlined up to this point should resolve most, if not all, causes of code P00AF, but if the code persists despite all attempts to resolve it by using these steps, the cause is likely to involve the turbocharger itself, the PCM, or the boost control module.

While the turbocharger’s boost level can be checked with a dedicated boost gauge, bear in mind that repairing the turbocharger (should there be something wrong with it), is best left to a specialist repair shop. Turbocharger failures are not always accompanied by mechanical noises, excessive smoke from the tail pipe, or even excessive oil in the inlet tract, so if a turbocharger failure is suspected, refer the vehicle to a specialist for professional diagnosis, and repair.

Bear in mind that there is little point in replacing any controller until it has been confirmed that there are no defects in the turbocharger itself, meaning that no controller should be replaced before the turbocharger has been shown to produce its design boost pressure throughout the engines’ operating range.

  • P00B0 – Relates to “Turbocharger/Supercharger Boost Control “B” Module Performance”

Note that on most applications, code P00B0 refers to issues with the turbo boost control related software, defects in/of systems drivers in the PCM, or modifications of the emission control system that cause compatibility issues with factory-installed software.

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