P0308 Cylinder #8 Misfire Detected

P0308 – Cylinder #8 Misfire Detected means the engine control module (ECM/PCM) has detected that cylinder number 8 is not producing combustion normally.

The important point is that P0308 identifies the cylinder where the misfire is being detected, not the component causing it. The fault could be related to ignition, fuel delivery, air supply, compression, valve timing, wiring, or even a mechanical problem inside the engine.

The ECM normally detects a misfire by monitoring small changes in crankshaft rotational speed. When cylinder #8 fails to produce its expected amount of power during combustion, the crankshaft slows slightly more than expected, and the control module recognizes this irregularity.

What Happens When Cylinder #8 Misfires?

A healthy engine produces relatively even combustion pulses from all cylinders. If cylinder #8 occasionally fails to ignite the air-fuel mixture, the engine's rotational speed becomes less consistent.

Depending on how severe the misfire is, the driver may notice:

• Rough or shaky idle

• Engine vibration

• Hesitation during acceleration

• Reduced engine power

• Poor throttle response

• Increased fuel consumption

• Engine shaking under load

• Difficult starting

• Intermittent stalling

• Check Engine Light

• Flashing Check Engine Light during severe misfire

A light misfire may be difficult to notice while driving, especially if it occurs only under acceleration. A severe P0308 condition, however, can make the engine run very roughly.

The Most Common Causes of P0308

The first things to investigate are the components that directly affect combustion in cylinder #8.

Ignition system

A worn or damaged spark plug is one of the simplest possibilities. The plug may have excessive gap, carbon deposits, oil contamination, cracked ceramic, or another ignition-related problem.

The ignition coil is another common cause. A coil can sometimes work normally at idle but fail when the engine is under heavy load. During acceleration, cylinder pressure increases and the ignition system has to generate a stronger spark. A weak coil may therefore produce a P0308 only under load.

The coil boot, high-voltage wiring on older ignition systems, and electrical connector should also be inspected.

Fuel injector

Cylinder #8 may not be receiving the correct amount of fuel.

Possible problems include:

• A clogged injector

• Injector internal failure

• Poor injector spray pattern

• Injector wiring problem

• Loose or corroded connector

• Short or open circuit

• Insufficient fuel pressure

An injector can have an electrically normal resistance value and still have a mechanical problem, so resistance testing alone does not completely verify injector operation.

Air or vacuum leak

An intake leak close to cylinder #8 can create a locally lean mixture. The engine may then misfire even though the ignition system is functioning correctly.

This becomes particularly important when the misfire consistently remains on one cylinder.

An intake smoke test can be useful when a vacuum leak is suspected.

Low compression

If the ignition and fuel systems appear normal, mechanical compression should be investigated.

Cylinder #8 may have:

• Burned valve

• Damaged valve seat

• Worn piston rings

• Damaged piston

• Head-gasket problem

• Valve-train failure

• Excessive carbon deposits affecting valve operation

A compression test followed by a leak-down test can help determine whether the problem is inside the cylinder.

Can a Bad Ignition Coil Cause P0308?

Yes. In fact, the ignition coil should be one of the early suspects when P0308 occurs, particularly on engines that use an individual coil for each cylinder.

One useful diagnostic method is a coil swap test.

If the engine design allows it, the ignition coil from cylinder #8 can be moved to another cylinder. The codes should then be monitored.

For example, if P0308 changes to P0305 after moving the coil, the misfire has followed the coil. That provides strong evidence that the coil is defective.

However, this test should be performed carefully because some ignition systems use different coil designs or cylinder-specific configurations.

Do Not Automatically Replace the Spark Plug or Coil

P0308 does not mean:

"Cylinder #8 ignition coil is bad."

It only means:

"The ECM detected abnormal combustion associated with cylinder #8."

This distinction is important.

Replacing parts one after another without testing can become expensive, especially when the actual problem is an injector, wiring fault, compression problem, or valve-train issue.

A good diagnosis follows the misfire instead of simply replacing the most common component.

Checking the Spark Plug

Remove the spark plug from cylinder #8 and compare it with the other cylinders.

Its appearance can provide useful information.

A fuel-wet plug may indicate that fuel is entering the cylinder but is not being burned correctly. A heavily carbonized plug can indicate an ignition or mixture problem. An oil-fouled plug can point toward an internal engine problem.

The plug gap should also be checked against the manufacturer's specification.

If the spark plug is worn, damaged, or contaminated, replacement may solve the problem. But if a new plug quickly becomes contaminated again, the underlying cause must still be found.

Checking the Ignition Coil

The ignition coil should be tested under the conditions in which the misfire occurs.

This is important because a coil can pass a basic electrical test and still fail under load.

Check:

• Coil power supply

• Ground

• Control signal

• Connector condition

• Terminal tension

• Wiring continuity

• Voltage drop

• Coil operation under load

An oscilloscope can provide much more information than a simple resistance measurement on some ignition systems.

If the coil can be swapped with another cylinder, monitoring whether the misfire follows the coil is often an effective diagnostic approach.

Checking the Fuel Injector

If the ignition system checks out, cylinder #8's injector becomes an important suspect.

Listen for injector operation where appropriate, inspect its electrical connection, and compare injector data with the other cylinders.

Depending on the vehicle, more advanced testing may include:

• Injector current waveform

• Injector balance testing

• Fuel pressure testing

• Fuel pressure under load

• Injector flow testing

A fuel pressure problem should not be ignored either. Low fuel pressure can cause misfires, although a problem affecting only cylinder #8 is more suggestive of a cylinder-specific fuel or ignition problem.

Compression and Leak-Down Testing

If the spark and fuel systems are functioning correctly but P0308 remains, mechanical testing becomes much more important.

A compression test can show whether cylinder #8 is producing adequate compression compared with the other cylinders.

If compression is low, a leak-down test can help identify where the pressure is escaping.

Air escaping through the intake can point toward an intake valve problem. Air escaping through the exhaust can indicate an exhaust valve problem. Air escaping into the crankcase can indicate piston ring or piston damage.

If coolant-system pressurization or other evidence is present, a head-gasket or cylinder-head problem may also need to be considered.

Wiring and Connector Problems

Electrical faults can produce a P0308 even when the spark plug, coil, and injector themselves are good.

Inspect the wiring around cylinder #8 for:

• Broken wires

• Chafing

• Melted insulation

• Corrosion

• Loose terminals

• Oil contamination

• Water intrusion

• Poor connector contact

A wiring problem can be especially difficult when the misfire occurs only during acceleration because engine movement can change the position of the harness.

A wiggle test while monitoring misfire data can sometimes reveal an intermittent connection.

Could Engine Timing Cause P0308?

Yes, but timing should not be blamed immediately when only P0308 is present.

Problems involving the timing chain, camshaft position, variable valve timing (VVT), or cam/crank synchronization can affect combustion.

If timing is the problem, however, there are often additional clues such as:

• Multiple-cylinder misfires

• Camshaft position codes

• Crankshaft position codes

• Cam/crank correlation codes

• Poor starting

• Reduced engine performance

• Abnormal timing-related live data

If only cylinder #8 is consistently misfiring and the ignition and injector tests point toward that cylinder, a cylinder-specific fault should generally be investigated first.

Use the Freeze-Frame Data

A diagnostic scanner can provide valuable information when P0308 is stored.

Look at the conditions under which the ECM detected the misfire:

• Engine RPM

• Engine load

• Coolant temperature

• Intake air temperature

• Fuel trims

• Throttle position

• Vehicle speed

• Misfire counters

For example, if cylinder #8 misfires only during hard acceleration, ignition or fuel delivery under load becomes particularly interesting.

If it misfires primarily at idle, an intake leak or other mixture-related issue may deserve more attention.

If the misfire occurs only when the engine is cold or hot, temperature-related ignition, fuel, wiring, or mechanical problems should be considered.

What If P0308 Appears Together With Other Codes?

Additional codes can dramatically change the diagnostic direction.

For example, P0308 together with other cylinder-specific misfire codes may indicate a broader ignition, fuel, mixture, or mechanical problem.

P0308 together with an ignition-coil circuit code points toward the ignition circuit.

P0308 together with fuel-pressure-related codes makes fuel delivery more important.

P0308 together with camshaft or crankshaft correlation codes raises suspicion of a timing or synchronization problem.

Therefore, it is better to diagnose all stored codes and freeze-frame information together rather than treating P0308 in isolation.

What About the Catalytic Converter?

A severe misfire should not be ignored.

When cylinder #8 fails to burn the fuel correctly, unburned fuel can enter the exhaust system. Excessive unburned fuel can overheat and damage the catalytic converter.

This is why a flashing Check Engine Light should be taken seriously. A flashing light generally indicates that the misfire is severe enough that continued operation could cause catalyst damage.

Avoid unnecessary driving until the cause has been diagnosed.

A Practical Diagnostic Order for P0308

A sensible diagnosis can proceed from the easiest cylinder-specific checks toward more advanced testing:

1. Read all stored and pending DTCs.

2. Check freeze-frame and misfire counters.

3. Inspect cylinder #8 spark plug.

4. Test or swap the ignition coil if applicable.

5. Inspect the coil and injector wiring.

6. Check injector operation and fuel delivery.

7. Look for intake or vacuum leaks.

8. Compare cylinder #8 compression with the other cylinders.

9. Perform a leak-down test if compression is abnormal.

10. Investigate valve timing, VVT, or cam/crank synchronization when supported by the evidence.

11. Consider ECM/PCM problems only after the external causes have been properly eliminated.

P0308 Does Not Automatically Mean Cylinder #8 Is Mechanically Damaged

This is an important distinction.

A misfire code can be caused by something as simple as a worn spark plug or poor electrical connection. It can also be caused by a failed injector or ignition coil. In other cases, the underlying problem may indeed be mechanical.

The code itself does not tell you which category is responsible.

The most useful question is therefore not "What part causes P0308?" but rather "What is different about cylinder #8 compared with the cylinders that are operating normally?"

That comparison often makes the diagnosis much faster.

Final Diagnostic Summary

P0308 – Cylinder #8 Misfire Detected means the ECM has identified abnormal combustion associated with cylinder #8.

The most common areas to investigate are the spark plug, ignition coil, injector, wiring, intake leaks, fuel delivery, and cylinder compression. If these systems are functioning correctly, valve-train operation, engine timing, VVT, and other mechanical causes should be considered.

The cylinder number itself must also be confirmed using the vehicle manufacturer's service information. On some engines, cylinder numbering and component locations are not intuitive, so assuming the physical location of cylinder #8 can lead to diagnosing the wrong component.

The best repair is not simply to clear P0308 or replace the most common part. Identify why cylinder #8 is producing a different combustion result from the other cylinders, then repair the underlying cause.