P0308 Code: Cylinder 8 Misfire Detected – Causes, Symptoms & Fix

P0308 is a generic OBD-II powertrain diagnostic trouble code (DTC) indicating that the Powertrain Control Module (PCM) has detected an engine misfire affecting Cylinder 8. A misfire occurs when the cylinder fails to produce its expected combustion contribution, resulting in uneven engine operation and reduced power.

The problem may originate from a worn spark plug, faulty ignition coil, malfunctioning fuel injector, wiring problem, insufficient fuel delivery, or internal engine damage. Accurate diagnosis requires identifying why cylinder 8 is not contributing properly rather than replacing components based on the trouble code alone.

What Does P0308 Mean?

• DTC: P0308
• Code Definition: Cylinder 8 Misfire Detected
• System: Powertrain / Engine Management
• Code Type: Generic OBD-II
• Monitored by: PCM / ECM
• Primary function monitored: Engine combustion consistency

The PCM identifies misfires by monitoring variations in crankshaft rotational speed through the crankshaft position sensor signal. When combustion fails or becomes weak, the crankshaft may experience a different acceleration pattern than expected. The PCM processes these variations and identifies the cylinder associated with the irregular combustion event.

P0308 does not identify a specific failed component. It identifies the cylinder where the misfire was detected.

Which Engines Can Set P0308?

P0308 applies to engines equipped with eight or more cylinders, including many V8 engine configurations. It can also apply to certain larger engine configurations containing a cylinder numbered 8.

Important: Cylinder numbering and firing order vary between manufacturers and engine designs. Always consult the vehicle-specific service information before identifying cylinder 8 or disconnecting ignition and fuel-system components.

Common Symptoms of P0308

• Check Engine Light illuminated or flashing.
• Rough engine idle and noticeable vibration.
• Engine hesitation during acceleration.
• Reduced engine power and poor throttle response.
• Increased fuel consumption.
• Unusual exhaust smell caused by incomplete combustion.
• Engine shaking under acceleration or heavy load.
• Difficulty starting in some cases.

A severe misfire can cause unburned fuel to enter the exhaust system, potentially overheating and damaging the catalytic converter.

What Causes Cylinder 8 Misfire?

1. Faulty Spark Plug

A worn, fouled, cracked, or incorrectly gapped spark plug may produce a weak or inconsistent spark. Oil contamination, carbon deposits, and excessive electrode wear can prevent reliable ignition inside cylinder 8.

2. Defective Ignition Coil

On engines using coil-on-plug ignition, the cylinder 8 ignition coil generates the high voltage needed to ignite the air-fuel mixture. An internally damaged coil, poor electrical connection, or insulation breakdown may cause intermittent or continuous misfires.

3. Fuel Injector Malfunction

A restricted, clogged, leaking, or electrically defective fuel injector can deliver an incorrect amount of fuel to cylinder 8. Both insufficient fuel delivery and excessive fuel delivery can interfere with combustion.

4. Ignition or Injector Wiring Problems

Damaged connectors, corroded terminals, loose wiring, poor grounds, or an open circuit can interrupt the ignition coil or injector control signals.

5. Low Cylinder Compression

Mechanical problems can prevent cylinder 8 from developing sufficient compression for proper combustion. Possible causes include:

• Burned or leaking intake or exhaust valves.
• Worn or damaged piston rings.
• Damaged piston or cylinder wall.
• Head gasket leakage.
• Valve-train or camshaft-related problems.

6. Intake Air or Vacuum Leakage

An intake manifold gasket leak or localized vacuum leak near cylinder 8 can introduce unmetered air and create an incorrect air-fuel mixture. This may be more noticeable at idle.

7. Fuel Pressure or Fuel Quality Problems

Insufficient fuel pressure, restricted fuel delivery, or contaminated fuel can contribute to misfires. If multiple cylinders misfire simultaneously, investigate common fuel-supply problems instead of assuming that several individual components have failed.

How to Repair P0308 – Cylinder 8 Misfire

Repairing P0308 requires identifying the actual cause of the cylinder 8 misfire. The following repairs should be considered only after completing the appropriate diagnostic tests.

1. Replace a Faulty Spark Plug

If inspection reveals worn electrodes, cracked ceramic, excessive deposits, or an incorrect spark plug gap, install the correct replacement specified for the engine. Check for oil or coolant contamination before installing the new plug.

2. Replace a Defective Ignition Coil

If a coil swap test confirms that the misfire moves to another cylinder, replace the defective ignition coil. Inspect the coil boot, connector, and electrical terminals for damage or corrosion.

3. Repair the Fuel Injector

If injector testing confirms restricted flow, leakage, or an electrical fault, clean or replace the injector as appropriate. Inspect the injector connector and wiring before installing replacement parts.

4. Repair Wiring and Electrical Connections

Repair damaged wiring, loose terminals, corroded connectors, or poor electrical grounds affecting the cylinder 8 ignition coil or fuel injector. Verify circuit operation after repairs.

5. Repair Mechanical Engine Problems

If compression or cylinder leak-down testing identifies leaking valves, damaged piston rings, or head gasket failure, perform the required mechanical repair. These problems may require cylinder head removal or engine disassembly.

Common Diagnostic and Repair Mistakes

Incorrect diagnosis can lead to unnecessary component replacement and increased repair costs. Avoid these common mistakes when troubleshooting P0308:

• Replacing the ignition coil immediately: P0308 does not prove that the coil has failed. Perform an ignition test or coil swap test first.
• Ignoring the spark plug: A worn or contaminated spark plug can cause the same cylinder-specific misfire.
• Replacing the fuel injector without testing: Check injector electrical operation and fuel delivery before replacement.
• Ignoring compression problems: Mechanical engine damage can produce persistent misfires even after installing new ignition components.
• Overlooking wiring damage: Corroded connectors and damaged harnesses may interrupt coil or injector operation.
• Ignoring freeze-frame data: The operating conditions when the fault occurred can help identify whether the misfire happens at idle, during acceleration, or under engine load.
• Replacing multiple components at once: Changing several parts without testing makes it difficult to identify the actual cause and wastes money.

P0308 Repair Cost – Estimated Prices

The cost of repairing P0308 depends on the failed component, vehicle model, engine configuration, labor rates, and parts availability. The following figures are approximate US repair estimates and may vary considerably.

Important: These are illustrative budgeting ranges, not guaranteed quotations. Diagnostic fees, taxes, additional damaged components, and engine-specific labor requirements may increase the total. Some V8 engines require substantial labor to access cylinder 8.

Is P0308 Dangerous? Driving Risks and Severity

P0308 can become a serious problem if the engine continues operating with an active misfire. The level of risk depends on misfire severity, operating conditions, and whether the Check Engine Light is flashing.

Can You Drive with P0308?

Driving with a flashing Check Engine Light is not recommended. A severe misfire can send unburned fuel into the exhaust system, causing excessive catalytic converter temperatures and potentially damaging the converter.

Other possible risks include:

• Reduced engine power during acceleration or overtaking.
• Unstable engine operation and excessive vibration.
• Increased fuel consumption and exhaust emissions.
• Potential catalytic converter overheating and damage.
• Engine stalling or hesitation in severe cases.
• Reduced ability to accelerate safely in traffic.

If the Check Engine Light flashes, reduce engine load, stop in a safe location, switch off the engine, and arrange professional diagnosis or recovery. Do not continue driving simply because the vehicle still moves.

If the light remains steady and the engine runs smoothly, arrange prompt inspection and avoid heavy acceleration, towing, and prolonged driving. If the engine shakes severely, loses power, or develops unusual noises, stop driving.

How Long Does It Take to Fix P0308?

Repair time depends on the confirmed cause and access to cylinder 8.

• Spark plug replacement: Approximately 30 minutes to 2 hours.
• Ignition coil replacement: Approximately 30 minutes to 2 hours.
• Injector diagnosis or replacement: Approximately 1–4 hours.
• Wiring diagnosis and repair: Approximately 1–3 hours or more.
• Internal engine repair: Several hours to multiple days, depending on the damage.

These are general workshop estimates. Some V8 engine layouts require additional labor to reach the rear cylinders.

How to Diagnose P0308 – Step-by-Step

Step 1: Scan the PCM for All Trouble Codes

Connect an OBD-II scanner and retrieve stored, pending, and permanent DTCs where supported.

Look for additional codes such as:

• P0300: Random or Multiple Cylinder Misfire Detected.
• P0358: Ignition Coil H Primary/Secondary Circuit.
• P0208: Injector Circuit/Open – Cylinder 8.
• P0171: System Too Lean Bank 1.
• P0174: System Too Lean Bank 2.

These codes can provide additional diagnostic direction, but their relevance depends on the engine configuration and manufacturer.

Record the freeze-frame information before clearing any codes. Pay attention to engine RPM, calculated load, coolant temperature, vehicle speed, and fuel trims.

Step 2: Check Cylinder 8 Spark Plug

Remove the spark plug using the correct service procedure and inspect its condition.

• Dry black deposits may indicate incomplete combustion or a rich condition.
• Wet fuel contamination may indicate that the cylinder is not igniting properly.
• Oil deposits may indicate an internal engine or valve-sealing problem.
• Cracked ceramic or worn electrodes can cause ignition failure.

Compare the cylinder 8 spark plug with other cylinders before deciding whether replacement is necessary.

Step 3: Perform an Ignition Coil Swap Test

If the engine uses individual ignition coils and the service procedure allows it, move the cylinder 8 coil to another cylinder.

Clear the codes only after recording the original diagnostic information, then operate the engine under the conditions that previously produced the misfire.

• If the misfire moves to the other cylinder, the ignition coil is a strong suspect.
• If P0308 remains, continue investigating the spark plug, wiring, injector, and mechanical condition.

This test should not be performed blindly on engines with different coil arrangements or special ignition systems.

Step 4: Test the Fuel Injector

Inspect the cylinder 8 injector connector and wiring. Check injector electrical operation using the manufacturer-approved procedure.

An injector balance test or suitable scan-tool actuator test can help determine whether the injector delivers fuel consistently compared with the other cylinders.

Remember that a normal injector resistance reading does not prove that the injector is mechanically delivering the correct fuel quantity.

Step 5: Perform a Compression Test

If ignition and fuel delivery tests do not identify the problem, compare cylinder 8 compression with the remaining cylinders.

A significantly lower reading may indicate leaking valves, worn piston rings, cylinder damage, or head gasket problems.

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

Step 6: Inspect Engine Mechanical and Control Systems

If ignition, fuel delivery, and compression are satisfactory, investigate possible valve-train problems, camshaft timing, intake leaks, electrical supply issues, and manufacturer-specific software or service bulletins.

P0308 Signal Flow – How the PCM Detects a Misfire

The following simplified signal-flow diagram explains how the engine control system monitors combustion and identifies cylinder 8 misfire activity.

Technical note: This is a functional monitoring diagram, not an electrical wiring diagram. The PCM uses crankshaft acceleration analysis and manufacturer-specific misfire-monitoring strategies to identify combustion irregularities.

P0308 Live Data – What Should You Monitor?

An advanced diagnostic scanner can provide useful information when investigating a cylinder-specific misfire.

• Cylinder 8 Misfire Counter: Monitor whether the misfire count increases during idle or acceleration.
• Engine RPM: Observe engine-speed stability during the misfire event.
• STFT: Short-Term Fuel Trim can indicate immediate fuel-mixture corrections.
• LTFT: Long-Term Fuel Trim helps identify persistent mixture corrections.
• Engine Coolant Temperature: Helps determine whether the problem occurs during cold or hot operation.
• Calculated Engine Load: Helps identify whether the misfire occurs under increased engine load.
• Mode $06: On supported vehicles, monitor cylinder-specific misfire test results and applicable monitoring limits.

Do not assume that a single fuel-trim reading identifies a cylinder 8 fault. Fuel trims generally reflect bank-level or system-level corrections, depending on the vehicle.

How to Fix P0308

The correct repair depends on the confirmed cause:

• Replace the cylinder 8 spark plug if it is worn or damaged.
• Replace a defective ignition coil after confirming the failure.
• Repair damaged ignition or injector wiring.
• Clean or replace a faulty fuel injector when testing confirms the problem.
• Repair intake manifold or vacuum leaks.
• Address low fuel pressure or fuel-quality problems.
• Repair internal engine damage if compression or leak-down testing confirms a mechanical fault.

After completing repairs, clear the codes when appropriate, perform a verification drive, and confirm that cylinder 8 misfire counts are no longer increasing.

Can You Drive with P0308?

Driving with P0308 is not recommended when the engine is actively misfiring.

If the Check Engine Light is flashing: Reduce engine load, stop driving as soon as it is safe, and arrange professional diagnosis or vehicle recovery. Continued operation can send unburned fuel into the exhaust and overheat the catalytic converter.

If the warning light is steady and the engine runs smoothly, arrange diagnosis promptly. Avoid heavy acceleration, towing, and prolonged driving until the fault is identified.

Related Trouble Codes

• P0300: Random or Multiple Cylinder Misfire Detected.
• P0301: Cylinder 1 Misfire Detected.
• P0302: Cylinder 2 Misfire Detected.
• P0303: Cylinder 3 Misfire Detected.
• P0304: Cylinder 4 Misfire Detected.
• P0305: Cylinder 5 Misfire Detected.
• P0306: Cylinder 6 Misfire Detected.
• P0307: Cylinder 7 Misfire Detected.

Frequently Asked Questions

Does P0308 always mean a bad ignition coil?

No. A faulty coil is one possible cause, but worn spark plugs, injector problems, wiring faults, vacuum leaks, and low compression can also produce this code.

Can a bad fuel injector cause P0308?

Yes. A clogged, leaking, or electrically defective cylinder 8 injector can prevent proper fuel delivery and cause incomplete combustion.

Can low compression trigger P0308?

Yes. Damaged valves, worn piston rings, cylinder-wall damage, or head gasket leakage can reduce compression and cause a cylinder-specific misfire.

Does P0308 affect fuel economy?

Yes. Incomplete combustion can reduce engine efficiency, increase fuel consumption, and increase exhaust emissions.

Can P0308 damage the catalytic converter?

Yes. A severe misfire can allow unburned fuel to enter the exhaust system, causing excessive catalytic converter temperatures and potential internal damage.

Where is cylinder 8 located?

Its physical location depends on the engine manufacturer and configuration. Consult the vehicle-specific cylinder numbering diagram before performing repairs.

OBDSpark Technical Note: P0308 identifies the cylinder where a misfire was detected, not the component responsible for it. Always compare ignition, fuel delivery, wiring, and mechanical test results before replacing parts.