2004 Chevrolet Impala P0305 and C0305: Misfire at Idle, Rough Acceleration, Check Engine Light Flashes Above 30 mph
The vehicle displays fault codes P0305 and C0305, indicating a cylinder misfire. The car runs roughly at idle, with intermittent smooth periods, and exhibits sluggish acceleration—feeling like one cylinder is not firing properly. When driving above 30 mph, the Check Engine light blinks intermittently; it stops blinking below that speed. Based on these symptoms, I’m considering three potential causes: 1. Clogged catalytic converter 2. Faulty fuel injector 3. Crankshaft position sensor malfunction The misfire is most noticeable during idle and low-speed driving, suggesting a problem with ignition or fuel delivery in cylinder 5.
For P0305, start by checking spark at the #5 plug using a spark tester. Verify injector pulse with an oscilloscope or diagnostic tool. Perform a fuel pressure test and monitor pressure drop during KOEO (Key On, Engine Off) to detect leaks. The #5 spark plug may show clues—wet or black? Also check for any signs of fuel in the engine oil, as this could indicate a serious internal issue.