P0308 – Cylinder 8 Misfire Detected means the engine control unit has identified that cylinder 8 is not firing properly. A single-cylinder misfire can cause rough idle, hesitation, reduced power, and higher fuel consumption. If ignored, the misfire can damage the catalytic converter due to unburned fuel entering the exhaust. This guide explains what P0308 means, its symptoms, causes, diagnosis steps, and the most effective repair solutions to restore proper engine performance.
What Does P0308 Mean?
P0308 is a generic OBD-II trouble code indicating that the ECU has detected repeated or significant misfires in cylinder 8. The ECU monitors crankshaft acceleration and cylinder contribution. When cylinder 8 consistently produces weak or incomplete combustion, the ECU stores P0308.
It is important to address this code quickly because misfires can lead to catalytic converter overheating and long-term engine damage if left unresolved.
Quick Reference
- OBD-II Family: P-Code (Powertrain)
- Scope: Generic
- System: Ignition / Fuel / Air / Mechanical
- Difficulty Level: Easy–Moderate
- Estimated Repair Cost: €50–€350+
- Last Updated: 5 December 2025
Real-World Example / Field Notes
A V8 sedan arrived with harsh vibration under load and a steady check engine light. Scan data showed a persistent misfire on cylinder 8. Inspection revealed an oil-fouled spark plug caused by a leaking valve cover gasket. Replacing the gasket, spark plug, and ignition coil resolved the issue.
Another case involved a large SUV with intermittent misfires only when towing. The injector for cylinder 8 had restricted flow at high load. Professional cleaning restored full injector performance and eliminated P0308.
Symptoms of P0308 – Cylinder 8 Misfire Detected
- Rough idle: Noticeable shaking or vibration.
- Loss of power: Reduced acceleration or hesitation.
- Poor fuel economy: Incomplete combustion wastes fuel.
- Check engine light: Blinking or steady depending on severity.
- Fuel smell: Unburned fuel may enter the exhaust.
- Stalling: Severe misfires can cause engine shutdown.
Common Causes of P0308
Most Common Causes
- Worn or fouled spark plug in cylinder 8
- Faulty ignition coil or damaged coil boot
- Clogged or leaking fuel injector (cylinder 8)
- Vacuum leak affecting cylinder 8 intake runner
- Dirty or failing mass airflow (MAF) sensor
Less Common Causes
- Low compression due to valve, piston, or ring wear
- Camshaft or crankshaft position sensor malfunction
- Fuel pressure too low due to pump or regulator problems
- ECU internal driver failure (rare)
- Carbon buildup causing poor combustion in cylinder 8
Diagnosis: Step-By-Step Guide
Diagnosing P0308 involves checking ignition components, fuel delivery, air metering, and mechanical engine condition.
Tools You’ll Need
OBD-II scan tool with misfire counters, spark tester, multimeter, fuel pressure gauge, compression tester, smoke machine, and basic hand tools.
- Check freeze-frame data: Identify when and under what conditions the misfire occurred.
- Inspect spark plug: Look for fouling, cracks, carbon buildup, or oil contamination.
- Swap ignition coils: Move coil 8 to another cylinder; if the misfire follows, the coil is faulty.
- Check fuel injector: Perform an injector balance test or swap injectors to identify flow problems.
- Inspect for vacuum leaks: Smoke-test the intake runner near cylinder 8.
- Clean or test MAF sensor: Incorrect airflow readings can lead to lean misfires.
- Review fuel trims: Lean trims suggest air leaks; rich trims may indicate injector leakage.
- Measure fuel pressure: Ensure pump and regulator are functioning properly.
- Perform a compression test: Low compression confirms mechanical issues affecting cylinder 8.
- Check timing correlation: Cam and crank timing errors can cause intermittent misfires.
Pro Tip: On V8 engines, rear cylinders like cylinder 8 often run hotter due to reduced airflow. Heat stress accelerates wear on ignition coils and plugs, so prioritize checking these components first.
Possible Fixes & Repair Costs
- Replace spark plug (cylinder 8): €20–€60
- Replace ignition coil: €50–€120
- Clean or replace fuel injector: €120–€250
- Repair vacuum leak: €50–€150
- Clean or replace MAF sensor: €20–€150
- Repair mechanical issues: €300–€900+
Can I Still Drive With P0308?
Driving with P0308 is not recommended. A misfire in cylinder 8 can cause unburned fuel to enter the catalytic converter, risking severe overheating and expensive damage. If the check engine light is flashing, stop driving immediately.
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Related Cylinder Misfire Codes
Compare nearby cylinder misfire trouble codes with similar definitions, fault patterns, and diagnostic paths.
Key Takeaways
- P0308 signals a misfire isolated to cylinder 8.
- Most common causes include spark plug, ignition coil, injector, or vacuum leak issues.
- A flashing MIL indicates severe catalyst-damaging misfires.
- Most repairs are affordable when diagnosed early.
FAQ
What is the most common cause of P0308?
A failing ignition coil or worn spark plug is the most frequent cause of cylinder 8 misfires.
Can a vacuum leak affect only cylinder 8?
Yes. An intake leak near the cylinder 8 runner can create a localized lean misfire.
Is it safe to drive with a cylinder 8 misfire?
No. Misfires can destroy the catalytic converter and lead to unsafe driving conditions.
How can I confirm a bad injector?
Use an injector balance test or swap injectors between cylinders to see if the misfire follows.
Should I replace all plugs or only one?
If spark plugs are old or worn, replacing all of them ensures balanced performance and reduces repeat failures.