2003 Chevy Venture P0303 Misfire and P0141 O2 Sensor Fault - How to Diagnose and Fix
My 2003 Chevrolet Venture is showing a flashing Check Engine Light with two fault codes: P0303 (cylinder #3 misfire) and P0141 (downstream oxygen sensor circuit malfunction). I've already replaced the downstream O2 sensor, but the issue persists. The vehicle exhibits a noticeable misfire, especially under load. I'm seeking reliable troubleshooting steps to identify the root cause of this problem. The cylinder #3 is located in the center rear bank. To diagnose further, I should first confirm if there's spark at cylinder #3. A wet or black plug typically indicates rich fuel mixture or a faulty injector, while a dry plug may suggest lack of fuel delivery. I've also performed a fuel pressure test under KOEO (Key On, Engine Off) conditions to check for leaks—this can help identify leaking injectors. Additionally, the vehicle makes a tapping sound, which my son believes could indicate a blown head gasket. However, recent maintenance in March included replacement of all head gaskets and head inspections, so this may not be the primary issue. I've checked oil and coolant levels—both appear normal—and conducted a basic inspection of the rear valve cover. Next steps include verifying spark at cylinder #3, confirming fuel injector pulses using a noid light, performing a compression test, and inspecting for any internal engine wear or debris in the oil.
A flashing check engine light is critical—cat converter damage is likely if not addressed. Focus first on resolving P0303 misfire, which occurs in cylinder #3 (center rear bank). Check for spark at cylinder #3. If no spark, investigate ignition components. Ensure the plug is wet and black; a dry plug may indicate fuel delivery issues. Perform a fuel pressure leak-down test under KOEO conditions to check for leaking injectors. This will help determine if an injector is faulty.