2004 Ford F-150 4.6L V8: P0351, P0356, and P0506 Fault Codes - Diagnosis & Fixes

Hello there. P0351: Ignition Coil A Primary/Secondary Circuit Malfunction (cylinder 1) P0356: Ignition Coil F Primary/Secondary Circuit Malfunction (cylinder 6) P0506: Idle Air Control System RPM Lower Than Expected A wiring diagram for the 2004 Ford F-150 4.6L V8 ignition system is available here: https://drive.google.com/file/d/12ptcUi...sp=sharing First, test spark at cylinders 1 and 6 by swapping known-good spark plugs into these positions to confirm if the issue is due to faulty coils or lack of spark. Possible causes include damaged high-tension (HT) leads, wiring faults in the shared coil pack harness (cylinders 1 and 6 share one wire due to wasted spark design), or a failing PCM. Start with this test and update your post if you find any spark issues or no ignition signal. For now, temporarily ignore P0506 as it may be secondary.

The 2004 F-150 4.6L V8 uses two separate ignition coil blocks (one for cylinders 1–4 and one for 5–8), each with a three-pin connector: the middle pin provides B+ power (12V), while the side pins carry switching signals from the PCM (switching negative). These signals must be verified using an automotive LED probe—standard bulb probes are not accurate. A recommended tool is available here: https://www.amazon.com/dp/B075S1QC6R/ref=dp_aws_pretitle?pf_rd_p=123456789&pf_rd_r=ABC123&pf_rd_s=aps Use this probe to check for proper signal presence at the coil connectors. This step is critical in diagnosing whether the issue lies with the coils or the PCM communication.

My truck has a coil-on-plug system, one ignition coil per cylinder—this configuration differs from older systems and applies to later model F-150s. However, the 2004 model likely uses dual coil packs with shared wiring for cylinders 1 and 6.

Apologies—I misidentified earlier due to confusion between coil-on-plug diagrams and older ignition systems. The 2004 F-150 4.6L V8 uses a dual-coil pack setup with shared wiring for cylinders 1 and 6. Could you please confirm if your truck shows any symptoms like misfiring or loss of power? Also, how many pins are on each ignition coil (2, 3, or 4)? The diagrams show two pins, but the actual connectors have three. Try lifting the coil pack slightly away from the plug while idling—can you hear a spark or observe visible sparking? Test both good and suspect coils to confirm whether the issue is with the coil itself or the wiring.

Got it—I’ll test cylinders 1 and 6 first. I’ve already replaced all eight plugs and all eight ignition coils, so I’m now focusing on the wiring harness and spark testing.

I do experience a noticeable lack of power at idle. The engine runs fine at higher speeds but misfires when returning to idle—this pattern suggests an idle control or ignition issue, possibly related to cylinder 1 or 6.

P0351 and P0356 point directly to issues with cylinders 1 and 6—either due to open/short circuits in the ignition coils, damaged wiring harnesses, or a faulty PCM. Since you've already replaced all coils and the codes remain, it's highly likely that the problem lies in the wiring harness, especially given the intermittent nature of the fault. I recommend inspecting the wiring between the engine and coil packs for any physical damage, rubbing, or corrosion.

Thanks! I’ll begin testing the wiring harnesses for cylinders 1 and 6 today.

Note: This post is unrelated to a Ford Fusion P0420 code. The original issue pertains only to the 2004 F-150 4.6L V8 ignition and idle system.