1999 Ford F250 V10 Misfire Diagnosis: P0300B, P0303, P0355 - Coil and Spark Plug Issues

The mechanic's assessment of coil health is questionable—ignition coils on Ford trucks are a common failure point. A carbon track in the coil boot often causes recurring misfires, and replacing only the coil without addressing the boot leads to early failure. The P0355 code (coil driver circuit malfunction) suggests either multiple coils have high resistance or there's an underlying issue with the PCM or wiring causing low voltage delivery to all coils. I recommend starting by replacing spark plugs with genuine Ford OEM parts, visually inspecting each coil, and using a digital multimeter (DVOM) to verify proper resistance values. If individual misfire codes are present, diagnosis becomes much clearer. Since Ford often only reports P0300-type codes, the ignition system—especially coils and plugs—is typically the first place to investigate. Good luck and please update on your progress. Jeff

The engine was replaced nearly three years ago with a 2001 V10, which has about 25K miles. Spark plugs were replaced at that time, so they now have only around 10K miles on them (I don't drive much). I've had to replace two ignition coils since then—both times due to similar symptoms of coolant leaking onto the coil. While it's unlikely, could the original 1999 engine control module (PCM) be faulty or incompatible with the newer V10 engine? This might explain inconsistent misfire behavior.

It's doubtful that codes are always accurate—misfires can sometimes appear sporadic. I know Ford recommends replacing all ignition coils when one begins to fail, though it can be expensive. Were the replacements full coils with boots, or just coil heads? What brand of spark plugs were used? These details matter for proper diagnosis.

The replacement coils were complete units including the boot, sourced from Ford. I believe the original spark plugs were AutoLite—non-OEM. The technician now recommends replacing all 10 ignition coils at $100 each and possibly the PCM at $500, totaling over $1,600. That seems excessive if the issue isn't truly widespread. Are there any more cost-effective or targeted solutions? Thanks for your input!

Replace only the ignition coils that have been identified as misfiring, install proper Ford OEM spark plugs, and avoid replacing the PCM unless absolutely necessary.

The original engine had 25K miles when it was installed. I've added about another 10K since then—would coils really need replacement at just 35K miles? This seems high for wear, especially if the vehicle is driven lightly.

Age can be as significant a factor as mileage. Unless you have access to a scope to confirm which cylinders are misfiring, it's hard to pinpoint exactly which ones are failing. I'd recommend starting with only the coils associated with current misfire codes and testing performance. This approach is safer and more cost-effective than replacing all 10 coils or upgrading the PCM.

I decided to replace just Cylinder #3's coil after removing it—there was visible coolant on the end of the coil. I used a compressor to blow out the passage, cleaned it with paper towels, and ensured it was completely dry. I then replaced the coil anyway since I had it already. Since that change, no misfire codes have appeared and the engine runs smoothly. Thanks for all the help!

You mentioned coolant on the tip of the coil—did you perform a pressure test on the cooling system to confirm the leak is fully resolved? If additional coolant enters the spark plug well, misfires will likely return.

Thanks for pointing that out. Yes, the cooling system was pressure tested when the initial leak was fixed—though I wasn't directly told how it was done. Only time will tell if the repair truly resolved the underlying issue.