Honda Civic 2000 LX Misfire Codes P1399, P0300-P0304: Causes and What to Check
I own a 2000 Honda Civic LX with 114,000 miles. The check engine light came on immediately after filling the tank with gas. I had previously added a bottle of Redline fuel injector cleaner before refueling. The warning light activated just a few yards after leaving the gas station. I checked the gas cap and confirmed it was securely tightened—listening for at least three clicks. To be thorough, I removed and reinstalled the gas cap, rotating it fully to ensure a tight seal. Despite this, the check engine light remains on. Using an OBD2 scanner, I retrieved codes: P0300-P0304 (misfire in cylinders 1–4) and P139-9 (manufacturer-controlled ignition system or misfire). According to online research: - P0301: Cylinder 1 misfiring - P0302: Cylinder 2 misfiring - P0303: Cylinder 3 misfiring - P0304: Cylinder 4 misfiring - P1399: Indicates a problem with the manufacturer-controlled ignition system or engine misfire. A local auto parts store representative suggested that clogged EGR valve ports could be contributing to the issue, possibly requiring cleaning or replacement. They also mentioned spark plugs as a potential cause. As a new car owner, I'm unsure what components to ask my mechanic to inspect. What specific parts should I request for diagnosis? Any advice would be greatly appreciated.
The misfire codes you've identified—P0300-P0304 and P1399—are clear indicators that a professional diagnostic is needed. These codes suggest issues with ignition system components or engine combustion. If you specifically request the mechanic to inspect or replace certain parts (like spark plugs, EGR valve, or fuel injectors) based on your assumptions, and those actions don’t resolve the issue, then you may have limited recourse. It's best to let the mechanic perform a full diagnostic using proper tools and procedures. Hold them accountable for identifying and fixing the root cause of the problem.