1998 Neon P0201, P0202, P0302, P0300 - How to Diagnose and Fix Common OBD-II Codes
I recently purchased a 1998 Neon with persistent OBD-II fault codes: P0201, P0202, P0302, and P0300. I decided to troubleshoot the issue myself using an ACTRON Auto Troubleshooter DVOM (Digital Volt-Ohm Meter). After reviewing the vehicle's service manual and understanding how the DVOM works, I'm confident in my ability to perform basic electrical diagnostics. I initially planned to take it to a mechanic, but I wanted to try diagnosing it myself first—especially since I have solid experience with internal engine components. However, I've found that the electronic systems can be tricky without proper tools and knowledge. Today, I disconnected the battery for two minutes and reconnected it. After running the engine for 15 minutes, the check engine light turned off, and idle quality improved noticeably. This suggests a possible reset of the ECU memory or a temporary clearing of stored fault codes. That said, I never disconnected the battery before buying the car—these codes were present from day one. The only code I currently see is P043 (misfire detected in cylinder #2). I've tested all injector connections and confirmed they are within specification. Further testing revealed that when I removed spark plug wires, all cylinders responded except for cylinder #2—which still shows no response despite receiving proper ignition from the coil. This strongly indicates a faulty injector or wiring issue in cylinder #2. I plan to replace all injectors next week and will perform a cut-out test after installation to verify the fix. I've also installed a cold air intake, which seems to improve engine performance overall. Before replacing any injectors, I recommend swapping injector #2 with injector #3 and retesting—this can help determine if the issue is specific to one injector or a shared system fault.
I bought an ACTRON Auto Troubleshooter DVOM and plan to inspect the entire engine system tomorrow. Originally, I was going to take it to a shop, but I decided to try diagnosing it myself first. While I'm confident in my mechanical skills, I'm less experienced with electrical systems. After reading the DVOM manual, I believe I can handle this—fingers crossed!