P0303 Fault Diagnosis for 2005 GMC Sierra 4.8L Engine
Hi everyone, I'm new to this forum and currently troubleshooting a P0303 fault on my 2005 GMC Sierra with a 4.8L V6 engine. I've already replaced the spark plugs, ignition wires, coil pack, and fuel injectors for cylinder #3. A compression test showed no significant issues, but the vehicle still idles roughly, lacks power, and only throws the P0303 code when idle. I'm looking for advice on next steps to diagnose this misfire issue accurately. Any help would be greatly appreciated. Thanks, JR
Using an OBD2 scanner can help monitor key sensor readings such as ECT (Engine Coolant Temperature), MAF (Mass Air Flow), MAP (Manifold Absolute Pressure), and oxygen sensor voltages—especially for Bank 1. If you don't have a scanner, the diagnosis will take longer. Ensure cylinder #3 is actually firing by checking spark plug activity. Are your plugs AC Delco? What's their condition—wet, dry, black, or tan? Perform a vacuum leak test, focusing on connections and fittings related to cylinder #3. Also conduct a fuel pressure test—what PSI are you reading? Use a noid light to check the #3 injector: does it flash (indicating intermittent signal), stay steady, or show no response at all? Could you share your previous compression readings for reference?