A 2012 Mercedes A-Class (W176) came into a Swedish workshop recently with a very clear complaint: the engine knocks under acceleration. The technician scanned the vehicle and pulled a single active fault code, P025218.
Diagnosing a knocking engine often sends mechanics down a lengthy path of checking injectors, measuring mechanical timing, or inspecting bottom-end bearings. However, this specific code completely redirects the diagnostic focus toward the high-pressure fuel delivery system.
Connecting P025218 to a physical knock
Code P025218 indicates an overcurrent condition. Specifically, it points directly to an electrical fault in the fuel metering solenoid valve, designated Y94, located on the high-pressure fuel pump for the OM651 engine.
It is easy to see an overcurrent code and assume a simple blown fuse or a dead relay is the culprit. But in this fuel system, the electrical fault creates a severe mechanical symptom. When the Y94 solenoid coil suffers an internal short circuit, it draws far too much current. This overcurrent state prevents the valve from correctly metering fuel into the high-pressure pump.
The immediate result is severe instability in the fuel rail pressure. When rail pressure fluctuates wildly during acceleration, the injection timing and injection pressure fall completely out of their required windows. That incorrect timing and pressure is exactly what causes the audible engine knock the customer complained about.
The diagnostic platform we build ranked an internal short in the Y94 valve as the most likely root cause for this exact vehicle and symptom combination, assigning it a 90 percent probability with high severity. To prove this in the bay, the technician needs to measure the electrical resistance across the Y94 solenoid coil and verify its mechanical response using an actuation test through their diagnostic tool.
Ruling out the wiring harness
Before ordering a replacement valve, the wiring harness requires a close look. Our engine ranked a wiring harness problem as the second most likely cause at 80 percent probability. A damaged, chafed, or shorted wire running between the ECU and the Y94 valve can directly cause the exact same overcurrent condition.
Heat-induced insulation failures or moisture intrusion at the connector pins are common culprits in this area of the engine bay. If the control signal to the valve is disrupted by a short circuit, the valve simply cannot regulate fuel properly. The rail pressure destabilises, and the engine knocks violently under load.
Verifying this cause involves a careful visual inspection of the harness routing, followed by a continuity test of the lines between the ECU and the valve. A short-to-ground or short-to-voltage test on those specific pins will confirm if the copper is compromised.
Mechanical binding disguised as electrical failure
Diagnostic codes only tell you what the computer sees. The ECU monitors current flow, so it flags an overcurrent code when the draw exceeds a threshold. But the root cause of that current spike might not actually be electrical at all. Our system ranked mechanical binding of the Y94 valve at 65 percent probability.
If the valve is jammed inside its housing due to dirt, metal particulates, or general mechanical wear, the solenoid will still attempt to move it. To overcome the physical resistance of the jammed valve, the solenoid pulls more and more current. The ECU sees this massive electrical spike and immediately sets P025218. Meanwhile, the valve remains stuck, failing to regulate fuel delivery, which keeps the rail pressure erratic and the engine knocking.
To separate a mechanical bind from a true electrical short, the technician should monitor the fuel pressure and the Y94 duty cycle in live data. If the duty cycle spikes aggressively but the pressure does not respond appropriately, the valve is likely jammed. A visual inspection of the valve after removal is the final confirmation step.
The high-pressure fuel pump variable
There is one more crucial layer to this fuel system behavior. Our diagnostic engine ranked a defective high-pressure fuel pump at 50 percent probability.
Internal wear or a mechanical failure within the high-pressure pump itself can cause insufficient pressure or inconsistent fuel delivery. When the pump fails to do its job, the metering valve tries to compensate by working far outside its normal operating range. This heavy compensation can cause the valve to draw excessive current, triggering the overcurrent code. The resulting instability in fuel delivery leads straight back to the knocking engine under acceleration.
Testing for this scenario requires a high-pressure side fuel test. The technician must monitor the requested fuel pressure versus the actual fuel pressure in the rail during a test drive or loaded acceleration. A significant gap between requested and actual pressure points to a pump failing to keep up with demand.
Next steps in the bay
In this Mercedes, the P025218 code zeroes the focus directly on the high-pressure pump and its metering valve. By systematically testing the Y94 resistance, checking the harness for chafing, and monitoring live duty cycle against rail pressure, a technician can isolate the exact point of failure without throwing unnecessary parts at the car. I would be interested to hear if you see this specific overcurrent and knock correlation on the OM651 engines in your own workshop.
Reference pages for the codes in this article
- P0252, what we measure for this code across real diagnostics