A 2009 Volkswagen Jetta with a 2.5L CBUA engine presented with persistent P0333 and P0328 fault codes, indicating a high circuit input issue with both knock sensors (K1 and K2). Despite replacing both sensors with FAE brand parts, ensuring proper torque (20NM) and applying a light silicone paste coating to the mounting surfaces, the codes returned.
Continuity tests confirmed no issues between the Powertrain Control Module (PCM) and the knock sensor connectors. Input, output, and ground shielding circuits all registered 0.5 ohms or less. PCM ground sources were also verified. The new knock sensors showed no shorts to the ground shield and exhibited a 4.7M ohm resistance for both K1 and K2.
Using VCDS Measuring Block 26 revealed unusual voltage readings. At idle, cylinders 1 and 5 displayed voltages between 0.650 and 0.700 volts. Under load at 1800 RPM, cylinder 2 voltage climbed to approximately 10 volts, while the other cylinders fluctuated between 5 and 7 volts.
The question remains: what are the expected knock sensor voltage specifications for this engine at different RPM ranges? What less common issues could cause a high input fault code despite replacing the sensors and verifying wiring integrity? The engine has high mileage (180,000) but exhibits no noticeable noise, and there are no loose brackets or components that could be impacting the block and generating false knock signals.
Possible causes beyond faulty sensors and wiring could include issues within the PCM itself, although less likely. Another potential factor is excessive carbon buildup within the cylinders, leading to pre-ignition and genuine knocking that the new sensors are accurately detecting. A thorough inspection of the engine for vacuum leaks or other performance issues that could indirectly affect knock sensor readings is also recommended.
Ultimately, determining the root cause requires comparing the observed knock sensor voltage readings with factory specifications available in official VW documentation or repair manuals. This information, coupled with further diagnostic testing guided by the potential causes outlined above, should pinpoint the problem and lead to a successful repair.
