← Back to list

P0308 Diagnostic for 2002 Lincoln Navigator: Spark Plug and Coil Replacement History

Model: Lincoln Navigator Fault Code: P0308 Posted: 2014-03-28 21:23

I own a 2002 Lincoln Navigator and recently received a P0308 diagnostic code. The vehicle runs smoothly with no noticeable engine misfire at idle—tachometer reads around 750 RPM, and only occasionally does the engine shake slightly without any change in tach reading. I replaced the spark plugs during my 100,000-mile service, and prior to that, the car was showing lean bank codes P0171 and P0174. After replacing the #8 cylinder coil (Motorcraft DG 543 model), the P0171/P0174 codes disappeared. However, shortly after, a new P0308 code appeared. I previously replaced a collapsed PCV elbow, which resolved the lean bank issues. The vehicle currently runs well with strong performance and an exhaust odor resembling sulfur. Despite no visible misfires or rough idle, the P0308 code persists. I've tried using premium fuel and adding Seafoam to the tank; after driving down to nearly empty, the P0308 code reappeared within minutes of idling. Given that ignition-related issues (spark plug or coil) are common causes for P0308, I'm now wondering whether further inspection is needed—specifically on the fuel injector or mechanical components. What should be my next troubleshooting step? Is it possible that the original diagnosis of a faulty coil was incorrect?

Related fault codes
P0308
Comments (4)
Anonymous 2014-03-29 10:15

Did you use the correct spark plugs? Try swapping the #8 cylinder's coil with another known-good coil to see if the P0308 code moves or disappears. This helps isolate whether the issue is ignition-related (coil or plug) versus fuel injection or mechanical problems. If the code stays on the same cylinder, it points more toward a faulty injector or internal engine component.

Anonymous 2014-03-29 10:45

I had my spark plugs replaced at a Ford dealership by a Lincoln technician with over 30 years of experience—using original Motorcraft plugs. The #8 coil was also replaced with a Motorcraft DG 543 unit. After driving about 20 miles, the 'Service Engine Soon' light returned. I took it to AutoZone and scanned the system—it showed P0171 and P0174. I then inspected for vacuum leaks and found one at the PCV elbow where it connects to the intake manifold. Replacing that hose cleared the lean codes temporarily. After another 20-mile drive, the check engine light returned, now showing only P0308. The vehicle runs smoothly with no shaking or rough idle, but there's a sulfur-like smell from the exhaust. I've tried premium fuel and added Seafoam; after driving down to nearly empty, the P0308 code appears within minutes of idling. What should I do next?

Anonymous 2014-03-29 11:57

The next logical step is to investigate the fuel injector or mechanical issues—since ignition components have been ruled out through replacement and testing. It would be advisable to return to the original technician and inform them that their repair did not resolve the issue, especially given the recurring P0308 code and persistent sulfur smell.

Anonymous 2014-03-29 14:56

Yes—since you paid for the service, it's reasonable to expect a proper diagnosis. If the problem persists after multiple repairs, consider seeking a second opinion or professional diagnostic equipment to verify the root cause of P0308.