P0354 is a generic OBD2 code that indicates that ignition coil “D” has an issue with the primary or secondary wiring circuits. If your Toyota Mirai has thrown this code, it may be accompanied by P0304. P0354 is a relatively straightforward diagnosis and is typically caused by a bad coil pack or wiring harness issue, although there can be caused by other factors.
P0354 Ignition Coil “D” Primary/Secondary Circuit → Malfunction
Primary or Secondary Wiring?
It’s important to determine if it is the primary or secondary circuit that has thrown the code. Luckily, it’s easy to determine which one is at fault.
The primary wiring is the wiring harness leading from your Mirai’s ECM/PCM to the ignition system itself. A short, open, or poorly ground wiring harness is almost always what causes P0354 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 is causing your Mirai to throw P0354. Ignition Coil “D” indicates the problem is in the fourth cylinder. You’ll need to look up which cylinder is number four on your engine. Different manufacturers use different methods. Here’s a pretty good explanation of how how to locate the fourth cylinder.
There is a really easy test to determine if it is the primary or secondary wiring that has thrown this code in your Mirai. Move the ignition components (coil pack, plug wire, plug) from cylinder D/4 to another and clear the codes. If the code “moves” from the fourth cylinder, you now know that it’s a secondary issue. If it stays, it’s a primary. We go into this in greater detail below (go straight there).
P0354 Symptoms: Toyota Mirai
- 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 with your Mirai.
- Misfiring– P0354 might be accompanied by P0304. P0304 indicates a misfire in the fourth cylinder. Resolving P0354 will typically clear P0304 as well. Misfiring can lead to a lack of power, and/or engine hesitation. It can also really mess up the catalytic converter.
- Rough Idle– You vehicle may idle rough if the fourth cylinder isn’t getting enough spark. You very well may smell raw fuel as your Mirai’s engine runs as well.
P0354 Causes: Toyota Mirai
These are the most common problems that cause P0354:
- Wiring Issues (short, open, or bad ground)
- Bad Coil Pack
- Loose Connection at the coil
- Bad PCM
The first goal of diagnosing P0354 is determining if you are dealing with a primary or secondary wiring problem in your Mirai. If you do that, you’ve eliminated half of the variables.
Voltage Test (Optional, but time saving)
The first place that you would want to start looking for what could be causing P0354 in your Mirai 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. If it has no signal, it’s time to look at the wiring harness. If you don’t have a voltmeter, you can use the parts swap method to help you determine if it’s the primary or secondary side causing your problems. See directly below ↓.
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 “jumps” to the cylinder that you’ve just swapped your Mirai’s 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 4. Start with the coil pack.
This method is only as difficult as the fourth cylinder is to get to.
If the misfire failed to move from the fourth cylinder, then you know that you most likely have an issue with the wiring harness going from the coil pack to the Mirai’s PCM/ECM. 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. Look to see if it looks burnt anywhere. 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, and an area where it may be time to call in the pro’s.
Wrapping Up: P0354 Toyota Mirai
P0354 is usually pretty easy to diagnose through the swap test, even if you don’t have any specialty diagnostic tools. If there is anything you can add, please leave a comment below. Good luck fixing your Mirai!