2009 Toyota Corolla LE 4-Cylinder P0301, P0304, CT0121 - Check Engine & VSC Light On at 100,000 Miles
Hi everyone, I'm looking for help with my 2009 Toyota Corolla LE 4-cylinder. The check engine light and VSC (Vehicle Stability Control) light have been flashing for the past few months. The issue started around 100,000 miles — at that point, the car began idling roughly when coming to a stop. Last year, I experienced acceleration problems, which led me to replace the ignition coils. After that, I took the car to a mechanic who recommended replacing the catalytic converter, which I did. However, the idle issue persisted. Recently, during a diagnostic check, the vehicle triggered P0301 (Cylinder 1 Misfire), P0304 (Cylinder 4 Misfire), and CT0120 (Catalyst Temperature Sensor Circuit Malfunction) codes. I've since replaced the spark plugs as well. Despite these efforts, the problem remains. The car still has a rough idle and the check engine light stays on. What could be causing this? Are there any other components or diagnostics I should consider? Any insights or suggestions would be greatly appreciated.
There are several potential causes for P0301 and P0304 misfire codes in a 2009 Toyota Corolla LE. A thorough diagnostic is essential to identify the root cause. Possible issues include: - Faulty spark plugs or ignition wires - Defective coil pack (especially for cylinders 1 and 4) - Malfunctioning oxygen sensors - Fuel injector problems in cylinders 1 or 4 - Burned exhaust valves - Catalytic converter failure (though CT0121 may indicate a sensor issue rather than full failure) - Low fuel pressure or poor fuel quality - Reduced engine compression - Faulty engine control module (ECM) It's important to note that P0301 and P0304 often point to misfires in specific cylinders, which can be caused by a combination of ignition, fuel delivery, or mechanical issues. A professional scan tool with live data monitoring is recommended to confirm the actual cause. If you haven't already, consider having a certified technician perform a full diagnostic including cylinder compression tests and fuel injector performance checks.