Experiencing a drivetrain malfunction warning in your BMW, accompanied by the fault code 120308? If you’re a BMW owner, particularly one driving a model with the N55 engine, this code might be causing you some concern. Fault code 120308 points to “Charging pressure control, plausibility Pressure too low,” indicating an issue within your engine’s turbocharging system. As an auto repair expert at obd-de.com, I’ll break down what this code means, potential causes, and how to approach troubleshooting.
This article is inspired by a real-world scenario from a BMW owner who encountered this exact problem. Like many enthusiasts, they started their journey by addressing other related issues, only to find themselves facing this low charging pressure fault. Let’s delve into the specifics of BMW fault code 120308 and equip you with the knowledge to tackle it effectively.
Understanding BMW Fault Code 120308
The error code 120308 in BMW vehicles with N55 engines signals that the engine control unit (ECU) has detected a discrepancy between the target and actual charging pressure within the intake system. Specifically, the system is reporting that the pressure is too low. This insufficient boost pressure can lead to reduced engine performance, hesitation, and the dreaded drivetrain malfunction warning light illuminating on your dashboard.
To understand this better, it’s crucial to know the basics of how the turbocharger system works in your BMW. The turbocharger forces more air into the engine cylinders, increasing power output. This “boost” is carefully regulated by the engine’s control systems, and fault code 120308 indicates a breakdown in this regulation, specifically on the low-pressure side.
Potential Causes of Code 120308
Several factors can contribute to a “Pressure too low” situation in your BMW’s charging system. Based on common issues and the experience of the BMW owner who initially reported this problem, here are the primary suspects:
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Boost Leaks: A leak in the intake system after the turbocharger is a prime suspect. Pressurized air escaping before reaching the engine will result in lower than expected pressure. Common leak points include:
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Charge Pipe Failure: The charge pipe, which carries pressurized air from the turbo to the intercooler and then to the intake manifold, is a known weak point in many BMW models. Cracks or breaks, especially in the plastic versions, are frequent occurrences. The user in the original post even mentioned replacing a broken charge pipe.
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Intercooler Hoses and Connections: Hoses connecting the intercooler and other components can become loose, cracked, or damaged over time, leading to leaks.
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Intake Manifold Gaskets: Less common, but intake manifold gasket leaks can also contribute to boost pressure loss.
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Turbocharger Wastegate Issues: The wastegate controls the turbocharger’s speed and boost output. If the wastegate is stuck open or not closing properly, it will bleed off exhaust gas and reduce turbocharger efficiency, resulting in low boost pressure.
- Wastegate Actuator Problems: The actuator, which controls the wastegate, can fail or malfunction, preventing proper wastegate operation.
- Wastegate Linkage Issues: The linkage connecting the actuator to the wastegate valve can become bent, loose, or disconnected. The user in the original post noted a “wiggling” wastegate, suggesting a potential linkage or bearing issue.
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Faulty Boost Pressure Sensor: A malfunctioning boost pressure sensor can provide inaccurate readings to the ECU. If the sensor incorrectly reports low pressure, the ECU will log code 120308, even if the actual pressure is within specification, or just slightly lower due to another issue.
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Vacuum Leaks in Control System: The turbocharger’s wastegate actuator is often controlled by vacuum. Leaks in the vacuum lines or components controlling the wastegate can disrupt its operation and lead to low boost.
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Oil Supply Issues to Turbocharger (Indirect): While less direct, insufficient or contaminated oil supply to the turbocharger can cause it to operate less efficiently over time, potentially contributing to underboost conditions in the long run, although this is less likely to be the primary cause of a sudden 120308 error. The user mentioned potential oil level issues, which might be a secondary factor to consider.
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ECU or Software Problems (Less Likely): In rare cases, a software glitch or ECU malfunction could lead to incorrect interpretation of sensor data or improper boost control. However, this is usually diagnosed after ruling out mechanical and sensor-related issues.
Diagnosing and Troubleshooting Code 120308
When faced with BMW fault code 120308, a systematic approach to diagnosis is crucial. Here’s a step-by-step guide, drawing from the troubleshooting steps hinted at in the original user’s post:
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Visual Inspection of the Intake System:
- Charge Pipe: Thoroughly inspect the charge pipe for cracks, splits, or loose connections. Pay close attention to the areas near the connections and any bends in the pipe. Upgrading to an aluminum charge pipe, as the original poster did, is a proactive step, especially for preventative maintenance on higher mileage N55 engines.
- Hoses and Clamps: Check all hoses and clamps in the intake system for tightness and signs of damage. Squeeze hoses to check for cracks or soft spots.
- Intercooler: Inspect the intercooler for damage, although leaks here are less common but still possible.
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Wastegate Examination:
- Manual Wastegate Check: Manually check the wastegate lever for smooth movement. A slightly wiggling wastegate, as mentioned by the user, might indicate wear in the linkage or wastegate shaft bearings. However, some minimal play can be normal. Excessive play or stiffness is a concern.
- Wastegate Actuator Function: If possible, use a vacuum pump to test the wastegate actuator. Apply vacuum and observe if the wastegate arm moves smoothly and holds vacuum.
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Boost Leak Test (Smoke Test): A smoke test is an effective way to identify leaks in the intake system. Introduce smoke into the system and look for smoke escaping from any points. This can pinpoint even small leaks that are hard to find visually.
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Live Data Diagnostics:
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Boost Pressure Monitoring: Use a diagnostic tool (like Delphi, ISTA-D if available) to monitor live data of the boost pressure sensor readings. Compare the actual boost pressure to the target boost pressure requested by the ECU, especially under load. The graphs provided by the original poster using Delphi diagnostics are excellent examples of how live data can reveal pressure discrepancies during different driving styles.
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Sensor Readings: Check the readings from the boost pressure sensor itself for plausibility. Compare it to other related sensor readings if available.
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Boost Pressure Sensor Test: If you suspect a faulty sensor, you can test it with a multimeter or by swapping it with a known good sensor (if available). However, verifying wiring and connections to the sensor is also crucial before replacing it.
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Vacuum System Check: Inspect all vacuum lines related to the turbocharger wastegate control for leaks, cracks, or disconnections. Use a vacuum gauge to check for proper vacuum levels.
Solutions and Repairs
The solution to BMW fault code 120308 depends entirely on the root cause identified during diagnosis. Here are potential repair paths:
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Boost Leak Repair:
- Charge Pipe Replacement: If the charge pipe is cracked, replace it. Consider upgrading to a more durable aluminum version.
- Hose and Clamp Replacement: Replace any damaged hoses or faulty clamps.
- Intake Manifold Gasket Replacement: If a leak is found in the intake manifold gasket, replace it.
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Wastegate Repair/Adjustment:
- Wastegate Actuator Replacement: If the actuator is faulty, replace it.
- Wastegate Linkage Adjustment/Repair: If the linkage is loose or bent, attempt adjustment or replacement. In some cases, if the wastegate flapper or turbo housing is excessively worn, turbocharger replacement might be necessary, although this is a more significant repair.
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Boost Pressure Sensor Replacement: If the sensor is confirmed to be faulty, replace it with a new, OEM-quality sensor.
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Vacuum Leak Repair: Replace any damaged vacuum lines or components.
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Software Update/ECU Check: In rare cases, a BMW dealer might recommend a software update or ECU reprogramming if other causes are ruled out.
Conclusion
Dealing with BMW fault code 120308 requires a methodical approach to diagnosis. Starting with a visual inspection and progressing to more detailed tests like smoke tests and live data analysis will help pinpoint the source of the low charging pressure issue. By systematically checking potential causes like boost leaks, wastegate problems, and sensor malfunctions, you can effectively troubleshoot and resolve this fault, restoring your BMW’s performance and eliminating that frustrating drivetrain warning. Remember to prioritize thorough diagnosis to avoid unnecessary parts replacements and ensure a lasting repair.
