2003 Chevrolet Silverado P0307 Misfire with Catalyst and EVAP Codes - How to Fix for Smog Pass?
I've taken my 2003 Chevrolet Silverado (4.8L Vortec engine) to three different repair shops, and despite two diagnoses, I still can't resolve the P0307 misfire code along with pending catalyst and EVAP codes. The truck runs smoothly under normal conditions, but it fails smog testing due to these codes. I've already replaced the spark plugs, had coil packs and fuel injectors checked — all deemed fine. Vacuum leaks were ruled out, and I replaced the intake gasket, which didn't resolve the issue. One shop suggested a faulty intake manifold, but after they reinstalled my original parts, I was told to return for further evaluation. I'm now frustrated because the truck runs well, yet it fails smog checks. The smog technician says the vehicle is 'not ready' due to pending catalyst and EVAP codes, which must be resolved before passing inspection. I've already added one can of Seafoam fuel system cleaner, but no improvement has been seen. I'm seeking advice on how to properly diagnose and fix this issue — especially since P0307 is a common misfire code that may stem from multiple underlying causes.
What are the exact OBD2 codes? Which engine model (e.g., 4.8L Vortec)? Is it stock or modified? P0307 indicates a misfire on cylinder #7 — possible causes include faulty spark, weak fuel delivery, or mechanical issues. Did you verify spark at the plug? How did the old #7 spark plug look — wet, dry, black, or oily? How were injectors tested? Have you checked compression? A full diagnostic with a capable OBD2 scanner is essential. Review freeze frame data for clues. Consider adding a fuel system cleaner like Techron (AC Delco recommended).