P0300 Fault in 2005 Dodge Neon SXT with California Emissions: Diagnosis and Fix

What brand of spark plugs and wires did you install? Dodge Neons can be sensitive to component quality.

I used Autolite copper spark plugs and AutoZone store-brand ignition wires.

After my last update, I replaced the ignition coils—no improvement. I now suspect either a fuel delivery issue or a sensor malfunction. The original pending codes P0301, P0003, and P0200 cleared after reassembling the fuel pump module. Could one of the injectors be sticking?

Older models are known to allow debris buildup in the fuel rail before injector failure—though I haven't seen it in a 2005 yet. I recommend visiting the dealer for OEM-quality plugs and wires, as I don’t trust AutoZone or Autolite for this application. Cheap Champion parts may suffice, but Neons are prone to carbon deposits. A revised heat range spark plug is available only through dealerships. Also verify EGR valve function and basic components like timing belt, compression, and fuel pressure.

The ignition coil module (where the spark plugs connect) could also be the culprit—worth checking.

Read the original post carefully. If you're not aware of what you're suggesting or are providing inaccurate advice, please refrain from posting.

Thank you for all the input! I’ve completed a compression test and other mechanical checks—all results are normal. During the compression test, cylinder #1 showed heavy carbon buildup; others appeared clean. After removing the injector rail and swapping injectors between cylinders #1 and #2, I drove for five days. Today’s plug inspection revealed that cylinder #2 had carbon deposits while cylinder #1 was clean. This suggests that the injector moved to #2 is leaking, causing the PCM to lean out the mixture in other cylinders—leading to misfires at idle. Should I replace only the faulty injector or a full set of injectors?

Before replacing fuel injectors (as spark plug color is not a reliable indicator), perform a fuel injector leak-down test. If this confirms that cylinder #2 is leaking, then replace just that injector. I believe it's more effective to clean the entire fuel rail thoroughly, replace the fuel filter, and only swap out the problematic injector. Only if all injectors are found to be faulty should you consider replacing them all. If budget allows, full replacement is an option.

I agree—replacing just the suspected injector may work. However, I’d recommend pressurizing the fuel rail with the rail lifted to check for dripping injectors and then actuating each injector to verify spray pattern before making a replacement decision. While early failure seems unlikely, contamination or poor fuel quality could be contributing factors. A misfire counter would help determine frequency and timing of misfires. Ensure the fuel rail is clean and properly maintained before installing any new injectors.

I plan to remove the injector rail for inspection. I’m already drilling and tapping it to allow future pressure testing during disassembly. The misfire worsens when coming to a stop in gear and resolves after a few seconds of idle in gear. Given that we recently refueled at a station off our usual route, fuel contamination is a plausible cause. Thanks for the helpful suggestions.

I purchased an injector from the dealer at the best price and installed it today. I also tapped a pressure test port on the end of the fuel rail where the factory omitted one. The vehicle now runs smoothly—no misfires, no check engine light. Thanks to everyone for their input!

*A collective 'you're welcome' from all contributors!*