2003 Buick Century 3.1L Engine P0301 and P0300 Codes - Diagnosis and Solutions
I own a 2003 Buick Century with a 3.1L engine, and I'm currently experiencing persistent OBD2 fault codes P0301 and P0300. I've already replaced the spark plugs, ignition wires, coils, ignition relay, throttle position sensor (TPS), fuel pressure regulator (FPR), fuel filter, and injectors. I also recently installed a new engine head and intake manifold gaskets, and the compression test shows good readings across all cylinders. Despite these repairs, the P0301 and P0300 codes persist. I'm wondering if this could be related to the oxygen sensor (O2 sensor), fuel pump performance, or both. Could a faulty O2 sensor or low fuel pressure be causing intermittent misfires? I've checked spark at cylinder #1, but I'd like more guidance on how to properly diagnose and resolve these issues.
Have you tested live data using an OBD2 scanner? Can you confirm the presence of spark at cylinder #1? Check the condition of the #1 plug—does it appear wet, dry, black, or tan? Aftermarket AC Delco plugs can sometimes cause misfires and trigger P0300 codes. What are your fuel pressure readings and KOEO (Key On, Engine Off) leak-down test results? A compression test was performed—what were the results? If all basic components have been ruled out: >> NOTE: In the 2003 Buick Century 3.1L engine, cylinder bank 1 (cylinders 1-3-5) is on the rear side, and cylinder bank 2 (cylinders 2-4-6) is on the front. >> A common issue is crossed fuel injector connectors. To verify, remove the intake manifold and inspect each circuit's color coding to ensure proper routing to the correct injector. A wiring diagram will be helpful for this step.