1999 Pontiac Bonneville Error Code P0304: How to Diagnose and Fix Ignition System Issues

NGK is an OEM supplier for many manufacturers, but that doesn't mean all aftermarket parts are equally reliable. While dealerships have never sold me faulty spark plugs in the past, I've encountered several questionable replacements from local auto part stores—parts that were labeled as 'acceptable' according to their catalogs. The term 'okay' here is key; it may not mean they're truly safe or effective. Given this, I recommend first checking the compression on cylinder #4, since a low reading could indicate mechanical issues rather than ignition-related problems. Additionally, try swapping the ignition wire from cylinder #4 to another cylinder to see if the error persists—this helps isolate whether the issue lies with the wire itself or the cylinder. I've never used this specific brand of ignition wire before, so I can't confirm its quality, but it's not uncommon for budget-brand wires to fail immediately upon installation. Have you already tried swapping the fuel injector on cylinder #4? And has fuel pressure been checked? Before focusing too much on spark plugs, it’s important to rule out other potential causes. Also, if moisture or water is present on ignition components—such as the spark plug or wires—does the engine misfire worsen significantly? This could point to a humidity-related issue that affects performance.