P0356 Explained: Ignition Coil 'F' Primary/Secondary Circuit Malfunction

Quick Answer

• What the code means: The P0356 code means that the engine control module (ECM) has detected a problem with the ignition coil for cylinder #6, or its circuit.
• Severity: Get this checked out as soon as possible. While you can likely drive home, continued driving can damage your catalytic converter, which is an expensive repair.
• Most Common Cause: The most common cause of a P0356 code is a faulty ignition coil for cylinder #6.

What the P0356 Code Means

The official OBD-II definition for P0356 is "Ignition Coil 'F' Primary/Secondary Circuit Malfunction." In plain English, this means the car's main computer, the ECM, has identified an issue with the ignition coil assigned to cylinder 'F', which is cylinder #6 on most vehicles.

The ignition coil's job is to take the 12-volt power from your battery and transform it into the 20,000+ volts needed to create a spark at the spark plug. This high-voltage spark is what ignites the air-fuel mixture in the cylinder, creating the combustion that powers your engine. The ECM continuously monitors the ignition coil circuits, and when it sees a voltage or resistance that's out of the expected range—either too high or too low—it triggers the P0356 code and turns on the Check Engine Light.

The 'F' in the code's name corresponds to the 6th cylinder in the engine's firing order. The terms 'primary' and 'secondary' refer to the two circuits within the ignition coil. The primary circuit is the low-voltage side that receives power from the battery, and the secondary circuit is the high-voltage side that delivers the spark to the spark plug. A fault in either of these circuits can trigger the P0356 code.

Common Symptoms

When the ignition coil for cylinder #6 isn't firing correctly, your engine's performance will suffer. You'll likely experience one or more of the following symptoms:

• Check Engine Light is on: This is often the first sign of a problem.
• Engine misfire: This will feel like a stumbling or hesitation from the engine, especially under load (like when you're accelerating).
• Rough idle: The engine may shake or vibrate more than usual when the car is stopped.
• Reduced engine power: You may notice that the car is slower to accelerate than it used to be.
• Worse fuel economy: With one cylinder not firing correctly, your engine has to work harder, which uses more fuel.
• Difficulty starting the engine: In some cases, a P0356 code can make the engine difficult to start.

Common Causes

While a number of issues can cause a P0356 code, some are much more common than others. Here are the most likely culprits, in order from most to least common:

1. Faulty ignition coil for cylinder #6: This is by far the most common cause. Ignition coils can fail due to age, heat, or vibration.
2. Damaged or corroded wiring or connectors: The wiring and connectors that link the ignition coil to the ECM and the spark plug can become damaged or corroded over time, leading to a poor connection and a P0356 code.
3. Worn-out or faulty spark plug in cylinder #6: A worn-out spark plug can put extra stress on the ignition coil, causing it to fail.
4. Vacuum leak: A vacuum leak near cylinder #6 can cause a lean air-fuel mixture, which can lead to a misfire and trigger a P0356 code.
5. Faulty Engine Control Module (ECM): This is the least likely cause, but it's still a possibility. If the ECM's internal circuits are damaged, it can mistakenly set a P0356 code.

How a Technician Diagnoses It

A qualified technician will follow a systematic process to diagnose a P0356 code. Here's a typical step-by-step diagnostic procedure:

1. Scan the vehicle for codes: The first step is to connect an OBD-II scanner to the vehicle's diagnostic port and confirm that P0356 is the only code present. If there are other codes, they'll need to be diagnosed as well.
2. Inspect the wiring and connectors: The technician will visually inspect the wiring and connectors for the ignition coil on cylinder #6, looking for any signs of damage, corrosion, or loose connections.
3. Check the spark plug: The technician will remove the spark plug from cylinder #6 and inspect it for wear, damage, or fouling. A good spark plug will have a light tan or grayish color on the insulator nose.
4. Test the ignition coil: The technician will test the ignition coil itself. They may do this by swapping the cylinder #6 coil with another one, say from cylinder #5. If the code follows the coil to the new cylinder (e.g., P0355), then the coil is bad. They might also use a multimeter to check the resistance of the primary and secondary circuits of the coil. Good values vary by manufacturer, but a professional will have access to the correct specifications.
5. Check for a vacuum leak: If the coil and spark plug are good, the technician will check for a vacuum leak near cylinder #6. This is often done with a smoke machine, which fills the intake manifold with smoke and makes any leaks easy to see.
6. Test the ECM: If all of the above tests come back negative, the technician will suspect a problem with the ECM. This is an advanced diagnostic step that requires specialized equipment.

Typical Repair Cost

The cost to repair a P0356 code can vary depending on the cause of the problem and the make and model of your vehicle. However, here's a general idea of what you can expect to pay:

• Ignition coil replacement: Parts can range from $75 to $300, and labor will typically be between $50 and $100. Total cost: $125–$400.
• Spark plug replacement: A single spark plug is inexpensive, around $10–$30. If you decide to replace all of them at the same time (which is recommended), the cost for parts will be higher. Labor is usually minimal if the coil is already being replaced. Total cost: $30–$200.
• Wiring repair: This can vary widely, from a simple $50 fix for a loose wire to several hundred dollars for a more complex repair to the wiring harness.

Can I Keep Driving With This Code?

It's not a good idea to keep driving with a P0356 code. While your car may still run, the engine misfire can cause serious and expensive damage to your catalytic converter. A new catalytic converter can cost over $2,000, so it's much more cost-effective to get the P0356 code diagnosed and repaired as soon as possible.

Related Codes

Several other codes are related to P0356, as they all have to do with the ignition system. If you have a P0356 code, you may also see one of the following:

• P0306: Cylinder 6 Misfire Detected
• P0350: Ignition Coil Primary/Secondary Circuit Malfunction
• P0351–P0355, P0357-P0362: These codes are similar to P0356, but they point to a problem with the ignition coil for a different cylinder.

If you're seeing a P0356 code, the best thing to do is to get your car to a qualified technician as soon as possible. Get this scanned at a trusted shop on Repairius.