P0358 is a generic OBD II diagnostic code that indicates an issue with the ignition coil “H” (it’s the eighth cylinder). Specifically with the primary or secondary wiring circuits.
If your Pontiac GTO has thrown P0358, it may be accompanied by P0308 (this code indicates that the eighth cylinder is misfiring). Addressing P0358 should take care of P0308.
P0358 is a relatively straightforward diagnosis and can be caused by a bad coil pack or wiring harness issue, although other less likely factors do exist in some cases. We cover the causes & diagnostic guidelines below.
P0358 Symptoms: Pontiac GTO
- 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 GTO.
- Misfiring– P0358 might be accompanied by P0308. P0308 indicates a misfire in the eighth cylinder. Resolving P0358 will typically clear P0308 as well. Misfiring can lead to a lack of power and/or engine hesitation. It can also damage the catalytic converter. If the check engine light is flashing, that means the misfire is so bad you shouldn’t be driving your GTO at all.
- Rough Idle– Your vehicle may idle rough if the eighth cylinder isn’t getting enough spark. You may smell raw fuel as the engine runs as well.
Primary VS Secondary Wiring
It’s essential 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. Use the swap test method listed directly after this section. You need almost no mechanical knowledge to perform this test, and it can really help you nail down what’s causing P0358 in your GTO.
The primary wiring is the wiring harness leading from your GTO’s ECM/PCM to the ignition system itself. A short, open, or poorly ground wiring harness is almost always what causes P0358 when the primary wiring side has thrown the code.
The secondary side of the equation is the spark side. The spark side is the plugs, plug wires (if so equipped), or a spark plugis causing your GTO to throw P0358.
Ignition Coil “H” indicates the problem is in the eighth cylinder.
You’ll need to look up which cylinder is number eight in your GTO’s engine. Different manufacturers use different methods. Here’s a pretty good explanation of how how to locate the eighth cylinder.
P0358 Diagnosis Tip “Swap Test”
There is a straightforward test to determine if the primary or secondary wiring has thrown this code in your GTO. First, move the ignition components (coil pack, plug wire, plug) from cylinder eight to another and clear the codes.
If the code moves to another cylinder, You now know it’s the spark plug, coil pack, or plug itself. You can keep doing this until you lock down the exact cause.
Here’s how it works: Now that we know it has to be one of the components you moved, move them again, but leave one behind. Then, once the code stops moving with the parts, you know you’ve found what’s causing P0358 in your GTO. It may be time-consuming, but it’s guaranteed to work if one of these components is causing the code.
P0358 Causes: Pontiac GTO
These are the most common problems that cause P0358:
- Wiring Issues (short, open, or bad ground)
- Bad Coil Pack
- Loose Connection at the coil
- Bad PCM
P0358 Diagnosis: Pontiac GTO
The first goal of diagnosing P0358 is determining if you are dealing with a primary or secondary wiring problem in your GTO. If you do that, you’ve eliminated half of the variables. You can do that by doing the swap test or using a voltmeter.
Voltage Test (Optional, but time-saving)
The first place that you would want to start looking for what could be causing P0358 in your GTO 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 (that’s the range for most coil packs).
If it was between 5-20hz, it’s doubtful that the coil pack has gone bad. However, it could 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 ↓.
Parts Swap Test
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 GTO’s ignition components into, then you know that what 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 the eighth cylinder. Start with the coil pack. The swap method is covered more in-depth in the section above.
If the misfire failed to move from cylinder eight, then you know that you most likely have an issue with the wiring harness going from the coil pack to your GTO’s PCM/ECM.
First, 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, you may need to replace the harness plug and pigtail.
If the misfire is intermittent, jiggle the wiring harness and see if it impacts how 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. PCM issues are the least likely answer and an area where it may be time to call in the pros.
Conclusion: P0358 Pontiac GTO
P0358 is usually pretty easy to diagnose through the swap test, even if you don’t have any special diagnostic tools. Good luck fixing your GTO!