P0351 → Ignition Coil “A” Primary/Secondary Circuit → Malfunction

P0351 is a generic OBD II trouble code.  it indicates that ignition coil “A” is malfunctioning.  It is typically caused by a bad coil pack or wiring harness issue. 

The primary wiring is the wiring harness leading from the PCM to the ignition system itself.  A short, open, or poorly ground wiring harness is almost always what causes P0351 when it is the primary wiring side that has thrown the code.

The secondary side of the equation is the spark side.  Which means that plugs, plug wires (if so equipped) or the spark plug itself are causing the code.  Ignition Coil “A” indicates the problem is in cylinder 1.  You’ll need to look up which cylinder is number one on your engine.  Different manufacturers use different methods.  Here’s a pretty good explanation of how how to locate cylinder 1.  


P0351 Causes + Fix

P0351 Symptoms

  • Check Engine Light– The check engine light may be all that you notice when you have this code, particularly if it is caused by an intermittent wiring issue.
  • Misfiring– P0351 might be accompanied by P0301.  P0301 indicates that there is a misfire in cylinder 1.  Resolving P0351 will typically clear P0301.  Misfiring can lead to a lack of power, and/or engine hesitation.
  • Rough Idle– You vehicle may idle rough if cylinder 1 isn’t getting enough spark.  You very well may smell raw fuel as the engine runs as well.


P0351 Common Causes

  • Wiring Issues (short, open, or bad ground)
  • Bad Coil Pack
  • Loose Connection at the coil
  • Bad PCM




Voltage Test (Optional, but time saving)

The first place that you would want to start looking for what could be causing P0351 in your vehicle would be the wiring harness.  You can use a digital voltmeter to check if the coil pack is getting the proper signal.  You would want to set it for A/C hertz and check to see if it is between 5-20hz. 

If it was between 5-20hz, it’s highly likely that the coil pack has gone bad.  Although, it could also be a bad plug or plug wire as well.  


Parts Swap

A great way to determine if it is an ignition related component, and not a wiring issue, is to reset the trouble code and swap the coil, plug, and plug wire (if equipped) with another cylinder.  If the misfire returns in the cylinder that you’ve just swapped your ignition components into, then you know that you are looking at is a coil, plug wire, or (most likely) coil pack failure.

If the misfire did move cylinders, you can go ahead and replace the plug, plug wire, and coil pack.  If you are on a tight budget, you can keep resetting the code and swap the ignition parts back one at a time until the misfire returns to cylinder one.  Start with the coil pack.



If the misfire failed to move from cylinder one, then you know that you most likely have an issue with the wiring harness going from the coil pack to the PCM.  Take a look at where the harness plugs into the coil pack.  Is it damaged or loose feeling in relation to the other cylinder’s connections?  If so it may need to be replaced.

If the misfire is intermittent, go ahead and jiggle the wiring harness and see if it has an impact on the way the engine is running.  If it does start or stop misfiring, you know you’ve found your problem.  Look anywhere that the harness bends or touches anything sharp.  Check for damage and repair the wiring accordingly.

Make sure that it has a solid ground.



If you still can’t find the problem, it’s time to take a look at the PCM.  This is the least likely answer.



P0351 is usually pretty easy to diagnose through the swap test, even if you don’t have any specialty diagnostic tools.  Good luck!