Dealing with a check engine light (CEL) in your 2008 Audi A6 4.2 can be frustrating, especially when it’s accompanied by OBD2 code P0408. This code often points towards issues with the exhaust gas recirculation (EGR) system, specifically the EGR Temperature Sensor “A” Circuit High. However, as many Audi owners discover, the problem can sometimes be more nuanced and involve related sensors like the Manifold Absolute Pressure (MAP) and Barometric Pressure (BARO) sensors.
One Audi owner’s experience highlights this complexity. Despite experiencing excellent engine performance and seemingly normal fuel economy, they were plagued by a persistent CEL and readings that suggested discrepancies between the MAP and BARO sensors. This individual, like many others, was hesitant to jump to drastic solutions like ECU replacement, especially when the car seemed to be running well.
The issue reported centers around sensor readings at key on engine off. The MAP sensor (G71) correctly reads ambient pressure, but the BARO sensor (F96) consistently shows a 91 millibar difference. This discrepancy is enough to trigger the CEL, even though the engine itself is performing optimally. The owner has already replaced the sensor multiple times, confirming with Bosch that they are using the correct dual MAP/BARO sensor. They have also conducted smoke and vacuum tests, and verified voltage readings, eliminating common culprits like vacuum leaks.
Possible areas to investigate further, based on this experience, include:
- Sensor Compatibility: While the owner confirmed the sensor with Bosch, double-checking part numbers and ensuring compatibility with a 2008 Audi A6 4.2 is always a good first step. Sometimes, even with the correct part number, subtle variations can cause issues.
- Wiring and Connections: A thorough inspection of the wiring harness and connectors leading to the MAP/BARO sensor is crucial. Look for any signs of damage, corrosion, or loose connections. Even minor wiring issues can lead to incorrect sensor readings.
- Reference Voltage: Verify that the sensor is receiving the correct reference voltage from the ECU. An analog meter can provide a basic check, but a digital multimeter or oscilloscope may be needed for more precise measurements.
- ECU Issue (Less Likely Initially): While ECU replacement is a significant undertaking, it cannot be entirely ruled out, especially if all other possibilities are exhausted. However, it should be considered as a last resort after meticulously checking all sensor-related components and wiring.
For owners facing a P0408 code in their 2008 Audi A6 4.2, particularly when sensor readings seem questionable, a systematic approach to diagnosis is key. Start with basic checks like sensor compatibility and wiring, and progressively move towards more complex diagnostics. While the frustration of a persistent CEL is understandable, methodically eliminating potential causes is the most effective way to resolve the issue and ensure your Audi is running smoothly and emissions-compliant.
Remember to consult repair manuals specific to your 2008 Audi A6 4.2 and consider seeking professional diagnostic assistance if you are not comfortable performing these checks yourself. Addressing sensor issues accurately is crucial for both engine performance and passing emissions tests.
