2002 Jeep Grand Cherokee Laredo 4.0L - P0351 Code After Coil Pack and Plug Replacement
My 2002 Jeep Grand Cherokee Laredo with a 4.0L engine originally showed codes P0302, P0305, and P0306. I replaced the coil pack and spark plugs (Champion copper core), which resolved those issues. However, after the replacement, the vehicle now displays code P0351. The truck previously shut off at around 50 mph and was towed home. It idles smoothly when stationary but shuts down after approximately 20 minutes of driving. After installing the new coil pack and plugs, it runs poorly—especially at idle, sounding like a helicopter. I've verified that all spark plugs are properly gapped, seated in the correct positions, and connected to the coil pack with secure harness connections. I tried re-installing the original coil pack, but the issue persists. I haven't yet replaced the old spark plugs (which I plan to try next). The only thing I didn- t do is test the original plugs—this may help identify if the problem lies in the ignition system or elsewhere. I'm concerned about needing a PCM replacement, as that would be costly and involve re-flashing. Any troubleshooting tips or diagnostics for P0351 on this model would be greatly appreciated.
To diagnose the P0351 code, disconnect the connector at the coil rail and use a test light connected to battery positive. Probe the tan/red wire (terminal #1) on the harness side while cranking the engine. The test light should blink—this checks the driver circuit of a ground-side switched transistor. If the test light does not blink, disconnect the PCM's C1 black connector and use an ohmmeter to measure resistance between terminal 1 (tan/red wire) at the coil rail connector and pin #7 on the PCM harness connector. Resistance should be below 5 ohms. Next, connect one probe of the ohmmeter to battery negative and the other to either pin #7 of the PCM harness or terminal #1 of the coil rail connector (with both connectors disconnected). You should read more than 5 ohms of resistance. If both resistance checks pass but the test light does not blink, a faulty PCM is likely. Before replacing the PCM, always verify power and ground connections at the PCM. A replacement will require re-flashing. Let me know your results—did you replace the coil rail? This information helps determine whether the issue lies in the ignition system or the control module.