If your check engine light just came on and your scanner shows P0308, you are dealing with a cylinder 8 misfire. It may sound technical, but the idea is simple. Your engine is not firing properly in one specific cylinder. That misfire can affect performance, fuel economy, and even cause long term damage if ignored. P0308 Code Explained: Causes, Symptoms, Fixes, and How to Prevent Engine Damage.
In this guide, I will walk you through everything you need to know about the P0308 trouble code in plain English. We will cover what it means, the most common causes, how serious it is, how to diagnose it, and how to fix it. Whether you drive a Ford F-150, Chevy Silverado, Dodge Ram, Toyota Tundra, or another V8 vehicle, this guide will help you understand what is going on under the hood.
What Does P0308 Mean?
The diagnostic trouble code P0308 stands for:
Cylinder 8 Misfire Detected
Modern vehicles use an onboard diagnostics system called OBD-II. When your engine control module detects that cylinder number 8 is not firing correctly, it stores this code and turns on the check engine light.
In simple terms, a misfire happens when the air and fuel mixture inside the cylinder does not burn properly. That could be because of a spark problem, fuel issue, compression issue, or even an air leak.
P0308 Code Explained: Causes, Symptoms, Fixes, and How to Prevent Engine Damage
Cylinder 8 is typically found in V8 engines. The exact location depends on the manufacturer, but it is usually the rear cylinder on one side of the engine.
Common Symptoms of a P0308 Code
A cylinder 8 misfire often comes with noticeable symptoms. Some drivers feel it immediately. Others only notice the check engine light.
Here are the most common signs:
- Check engine light on
- Rough idle
- Engine shaking or vibrating
- Loss of power
- Poor acceleration
- Reduced fuel economy
- Engine hesitation
- Strong fuel smell
- Flashing check engine light under load
If the check engine light is flashing, that means the misfire is severe enough to potentially damage the catalytic converter. In that case, avoid driving until the issue is fixed.
What Causes a Cylinder 8 Misfire?
There is no single cause for P0308. It can come from several different systems in your engine. Below are the most common causes.
1. Bad Spark Plug
Worn or fouled spark plugs are the most common reason for a misfire. Over time, spark plugs wear down and lose the ability to create a strong spark.
If cylinder 8 has an old or damaged spark plug, it may not ignite the air and fuel mixture properly.
2. Faulty Ignition Coil
Many modern vehicles use a coil-on-plug ignition system. That means each cylinder has its own ignition coil.
If the coil on cylinder 8 fails, it will not deliver the proper voltage to the spark plug. This is a very common cause of P0308.
3. Fuel Injector Problems
If the fuel injector for cylinder 8 is clogged or not working, the cylinder may not receive enough fuel.
Symptoms of a bad injector can include rough idle and poor performance, especially under load.
4. Vacuum Leak
A vacuum leak near cylinder 8 can create an imbalance in the air to fuel ratio. Too much air and not enough fuel can lead to a lean misfire.
5. Low Compression
Internal engine problems such as worn piston rings, damaged valves, or a blown head gasket can reduce compression in cylinder 8.
Low compression means the fuel and air mixture cannot ignite properly.
6. Wiring or Connector Issues
Sometimes the problem is electrical. Damaged wiring or loose connectors going to the ignition coil or injector can trigger a misfire.
7. Engine Timing Problems
In rare cases, incorrect valve timing or timing chain issues can cause cylinder specific misfires.
How Serious Is Code P0308?
A single cylinder misfire might not seem like a big deal, but it can become serious if ignored.
Here is why:
- Misfires waste fuel
- They increase emissions
- They reduce engine performance
- They can damage the catalytic converter
When fuel does not burn in the cylinder, it can enter the exhaust system and overheat the catalytic converter. Replacing a catalytic converter can cost over $1,000 in many vehicles.
If the check engine light is flashing, that means the misfire is severe and you should avoid driving until it is repaired.
How to Diagnose P0308
If you are comfortable working on your vehicle, you can perform some basic checks. Otherwise, a mechanic can diagnose it quickly.
Here is a logical way to approach it.
Step 1: Confirm the Code
Use an OBD-II scanner to verify P0308. Check if there are additional codes such as:
- P0300 random misfire
- P0171 lean condition
- Fuel injector related codes
Extra codes can provide helpful clues.
Step 2: Inspect the Spark Plug
Remove the spark plug from cylinder 8 and inspect it.
Look for:
- Heavy carbon buildup
- Oil contamination
- Cracked porcelain
- Excessive wear
If it looks worn, replace it.
Step 3: Swap Ignition Coils
This is a simple and effective test.
Swap the ignition coil from cylinder 8 with another cylinder. Clear the codes and drive the vehicle.
If the misfire moves to the new cylinder, the coil is faulty.
Step 4: Check the Fuel Injector
You can listen to the injector with a mechanic’s stethoscope. A clicking sound usually means it is functioning.
If needed, test injector resistance with a multimeter.
Step 5: Perform a Compression Test
If spark and fuel components check out, perform a compression test. Low compression in cylinder 8 indicates internal engine problems.
How to Fix P0308
The repair depends on the cause. Below are the most common fixes and estimated costs in the United States.
Replace Spark Plug
Cost: $10 to $25 per plug plus labor
If your plugs are worn, replacing all of them is often a good idea.
Replace Ignition Coil
Cost: $75 to $300 per coil depending on vehicle
This is one of the most common repairs for P0308.
Replace Fuel Injector
Cost: $150 to $500 per injector
Injector replacement can be more expensive depending on labor time.
Repair Vacuum Leak
Cost: $100 to $400
This depends on the location of the leak.
Engine Repair for Low Compression
Cost: $1,000 to $4,000 or more
If the problem is internal, such as worn valves or piston damage, repair costs increase significantly.
Can You Drive With a P0308 Code?
It depends on severity.
If the engine runs slightly rough but the light is steady, you may be able to drive short distances. However, it is not recommended for long periods.
If the check engine light is flashing, stop driving as soon as it is safe. Continuing to drive can destroy the catalytic converter.
Vehicles Commonly Affected by P0308
P0308 often appears in V8 engines, including:
- Ford F-150
- Chevrolet Silverado
- GMC Sierra
- Dodge Ram 1500
- Toyota Tundra
- Jeep Grand Cherokee V8 models
- Cadillac Escalade
The code is not limited to trucks. Any vehicle with eight cylinders can experience a cylinder 8 misfire.
Preventing Future Misfires
You can reduce the risk of P0308 by following a few simple maintenance practices.
Replace Spark Plugs on Schedule
Check your owner’s manual for recommended intervals. Many modern spark plugs last 60,000 to 100,000 miles.
Use Quality Fuel
Low quality fuel can cause carbon buildup and injector problems.
Address Check Engine Lights Early
Small issues can become big ones if ignored.
Keep Up With Regular Maintenance
Air filters, fuel filters, and oil changes all contribute to proper engine function.
Frequently Asked Questions
Is P0308 the same as P0300?
No. P0300 indicates random or multiple cylinder misfires. P0308 specifically points to cylinder number 8.
Will a tune up fix P0308?
If the misfire is caused by worn spark plugs or coils, a tune up may resolve it. If the issue is mechanical, additional repairs will be required.
Can bad gas cause a cylinder 8 misfire?
Yes. Contaminated or low quality fuel can cause misfires, though it often affects multiple cylinders.
How much does it cost to fix P0308?
It can range from under $100 for a spark plug replacement to several thousand dollars for major engine repairs.
Final Thoughts on P0308
A P0308 code means your engine is not firing properly in cylinder 8. In many cases, the fix is straightforward, such as replacing a spark plug or ignition coil. In more serious cases, it may point to deeper engine issues.
The key is not to ignore it. Misfires can lead to expensive repairs if left unresolved. If you feel shaking, hesitation, or see a flashing check engine light, have it inspected as soon as possible.
If you are comfortable with basic tools, you may be able to diagnose and fix the issue yourself. If not, a qualified mechanic can pinpoint the cause quickly.
Taking care of the problem early protects your engine, improves fuel economy, and keeps your vehicle running smoothly for years to come.