2003 GMC 5.3L Engine P0302 Flashing Misfire Code: Diagnosis and Fix
I own a 2003 GMC 5.3L V8 truck that sets up the P0302 flashing misfire code specifically when driving on highways at speeds between 50 to 60 mph. The check engine light flashes under these conditions, while on city roads it only stays solidly lit with occasional brief flashes. I've already replaced all spark plugs (NGK), plug wires, and coils. Fuel pressure is within normal range, vacuum system checks out, and injector resistance readings are consistent at 13.6 ohms — all within specifications. The issue appears to be intermittent: the engine starts and runs fine when cold, but after driving approximately 5 miles on highway conditions, the P030-2 code reappears and remains active until I clear it via a scan tool. The only thing I can currently suspect is a faulty O2 sensor for Bank 2 Sensor 1 or a malfunctioning fuel injector. I'm looking for advice from experienced owners — any insights into potential root causes, especially related to spark delivery or injector performance? This truck has accumulated 94,000 miles and I'd like to resolve this before further engine damage occurs.
The 2003 GMC 5.3L engine is known to be sensitive to spark plug quality — AC Delco plugs are recommended over other brands, as non-AC Delco types can trigger misfire codes like P0302. A flashing P0302 code indicates a serious issue that could lead to catalytic converter damage if not addressed promptly. Please confirm whether the #2 injector is showing a flash or steady signal using a noid light — is it receiving 12V? Is the driver wire properly grounded? Also, check if spark is present at cylinder #2. A lack of spark or weak ignition could be the root cause. This points to either a failed spark plug or an injector issue causing over-fueling.