2002 Chevrolet S10 V6 P0303 Misfire: Causes, Diagnosis & Solutions for Long Incline Driving

You're experiencing a misfire on cylinder #3, with no other diagnostic codes reported. First, confirm whether you used OEM or high-quality aftermarket parts—specifically AC-DELCO components—for your ignition system. If not, that should be your first step. The flashing check engine light strongly suggests potential catalytic converter damage. My initial suspicion is an injector over-fueling issue. Please inspect cylinder #3's spark plug: is it wet, dry, black, or tan in color? Perform a fuel pressure test and monitor the drop when the engine is turned off—this can help identify delivery issues. If you have a scan tool, check the fuel trim values for anomalies. As a last resort, try adding one can of SeaFoam fuel system cleaner to see if it clears up injector clogs or deposits.

I disagree that a clogged catalytic converter would cause only P0303. Typically, such issues result in broader misfires (like P0300), not isolated cylinder faults. I recommend replacing the distributor cap and rotor with genuine AC-DELCO or Wells parts—many cheaper aftermarket caps suffer from inductive crossfire due to poorly designed air ports in the base. There’s a helpful YouTube video by Mark J Hicks that demonstrates how these distributor designs can lead to repeated rotor failures and misfires. I highly recommend watching it for deeper insight into this issue.

Thanks for the tip! The video you mentioned is indeed on Mark J Hicks’ channel—great resource. Here’s a link: http://www.youtube.com/user/markjhicks. I replaced both the distributor cap and rotor, which were not OEM parts. I expected this to resolve the issue temporarily—but it hasn’t helped. I agree with your point about catalytic converter failure—it usually causes random misfires rather than isolated ones. The problem only occurs under load at 2500 RPM and 50 mph, so I suspect an injector issue. However, given the cost and difficulty of replacing injectors, I want to eliminate all other possibilities first.

Thanks for the feedback. The spark plug on cylinder #3 shows a white-tan color—similar to cylinder #5. Even though the parts weren’t OEM, I’d expect them to have provided some temporary relief. I can reliably reproduce this fault by driving up a local hill at 50 mph. I don’t currently own diagnostic tools to perform fuel pressure leak-down or fuel trim tests—but if such a test failed, what would that indicate? My current theory is that the issue isn't with fuel pressure upstream of the injector regulator, as that would affect all cylinders and likely trigger multiple misfire codes. Similarly, a catalytic converter failure wouldn’t be isolated to one cylinder. If it were a fuel delivery problem, I’d expect it to point directly to cylinder #3’s injector—so I’m leaning toward that direction. I’ll try the SeaFoam treatment as a simple test first.

I haven’t seen the exact video you’re referring to, but you're definitely on the right channel! I’d strongly recommend purchasing a genuine Wells or dealer-grade distributor cap and rotor. Reinstalling these components could resolve crossfire issues that may be contributing to misfires. I remain skeptical about replacing an injector without further testing. Without scan tools or gauges, it’s hard to confirm. Watching Mark J Hicks’ videos on GM truck engines might provide valuable context for diagnosing this specific model.

Injector replacement is not a simple task—especially without diagnostic equipment. Let's follow Jeff’s suggested approach. Can you reproduce the misfire without an incline? Try driving on flat ground with heavy throttle to isolate whether it's load-related. Double-check your secondary ignition system: inspect wiring, routing, insulation in looms, and separator components—this coil is known for arcing. While distributor issues were mentioned, they should have appeared earlier than 110,000 miles. Ultimately, this points back to basic troubleshooting—systematic elimination of common causes before moving toward expensive repairs.