A 2018 Mercedes-Benz S-Class (W222) arrived in the bay logging P00AF73 and P00AF72, jerking under load with a turbo actuator that refused to open. The technician was stuck on a simple question: which component is actuator A, and is the sensor dead or the unit itself? The car exhibited severe drivability issues, jerking noticeably when the driver demanded power. The codes explicitly stated that boost pressure control A had a malfunction and the actuator was not opening.
We see this pattern frequently in the diagnostic platform we build. A technician reads a code that points directly at a component, but the vehicle symptoms and the engine architecture demand a wider view of the system before ordering parts.
The problem with generic actuator codes
Fault codes P00AF72 and P00AF73 directly indicate a problem with the boost pressure controller. On the OM656.929 engine, “actuator A” refers specifically to the electronic variable geometry turbocharger (VNT) actuator mounted directly on the turbo assembly.
The diagnostic engine ranked an internal electronic failure of this VNT actuator as the most likely root cause, placing it at a 90 percent probability with a high severity rating. The internal gears of the actuator wear out over time, or the internal electronics fail completely. When this happens, the actuator cannot move the VNT vanes to regulate boost pressure. The resulting lack of air mass control causes the exact jerking symptom the technician noted in the job card.
Verifying this requires a scan tool to perform an actuator output test. You must also check the power and ground supplies directly at the actuator connector, followed by a thorough visual inspection for physical casing damage.
Mechanical binding versus electrical failure
The second most likely cause, ranked at 85 percent, is a mechanical failure of the turbocharger itself. Specifically, the variable geometry vanes get stuck.
Diesel engines accumulate heavy carbon soot. This soot packs into the turbo housing and jams the VNT vanes. When the vanes cannot move freely, the VNT actuator is physically blocked from modulating them. The engine control module sees the actuator failing to reach its commanded position and logs P00AF72 and P00AF73. Replacing an electronic actuator when the physical vanes are jammed will destroy the new actuator very quickly. The motor inside the new unit will overload trying to push against the immovable carbon.
To rule this out, you have to disconnect the actuator linkage and check the movement of the VNT rod manually. It must move completely freely without any gritty resistance. While you are there, a visual check of the turbocharger shaft for excessive radial and axial play is mandatory.
Wiring and communication faults
Before condemning either the turbo or the actuator, you have to consider the harness. The system ranked electrical wiring and connector problems at 70 percent.
Corroded, damaged, or loose wiring between the engine control module and the VNT actuator will interrupt the power supply or the control signal. This often results in intermittent operation, which aligns perfectly with a jerking sensation under acceleration. The fix here relies on basic electrical testing. You need a continuity test of the circuits, along with voltage and resistance measurements back to the engine control module.
The hidden exhaust restriction
The most deceptive cause on the list sits entirely downstream of the turbocharger. The system ranked a clogged diesel particulate filter (DPF) at 60 percent probability.
A severely restricted DPF creates immense exhaust backpressure. This backpressure pushes against the exhaust turbine, physically resisting the flow required to spool the turbocharger and create boost pressure. The engine control module registers the lack of boost, notes that the actuator is struggling to compensate against the exhaust pressure, and sets the P00AF7x codes.
In this scenario, the turbocharger and the actuator are perfectly healthy. They are just losing a physical fight against a blocked exhaust. You can verify this by monitoring the DPF differential pressure and the exhaust backpressure sensors during a test drive. If the restriction is high, attempting a forced regeneration of the DPF is the required next step before touching the boost control system.
What to check first
Do not fire the parts cannon at the turbo actuator just because the code definition mentions it. Disconnect the linkage and feel the vanes by hand. Test the electrical circuits, and verify your exhaust backpressure. Are you seeing similar boost control codes masking DPF restrictions in your own bays?
Reference pages for the codes in this article
- P00AF, what we measure for this code across real diagnostics
