P0309 – Cylinder 9 Misfire Detected means the engine control unit has detected a misfire occurring specifically in cylinder 9. A misfire can cause rough idle, hesitation, loss of power, and poor fuel economy. If ignored, unburned fuel may enter the exhaust and damage the catalytic converter. Cylinder 9 misfires may come from ignition failure, fuel delivery problems, airflow issues, or mechanical wear. This guide explains what P0309 means, its symptoms, causes, diagnosis steps, and the most effective repair solutions to restore proper engine operation.
What Does P0309 Mean?
P0309 is a generic OBD-II diagnostic code indicating that the ECU has detected repeated misfire events in cylinder 9. The ECU monitors crankshaft speed fluctuations and compares combustion contribution from each cylinder. When cylinder 9 produces weak or incomplete combustion, the ECU stores P0309.
This misfire should be repaired quickly because prolonged misfires allow unburned fuel to enter the exhaust, which can overheat and destroy the catalytic converter.
Quick Reference
- OBD-II Family: P-Code (Powertrain)
- Scope: Generic
- System: Ignition / Fuel / Air / Mechanical
- Difficulty Level: Moderate
- Estimated Repair Cost: €70–€400+
- Last Updated: 5 December 2025
Real-World Example / Field Notes
An industrial V9 power unit arrived with harsh vibration and poor throttle response. Cylinder balance testing showed cylinder 9 firing weakly. The spark plug was oil-fouled due to a worn valve stem seal. Replacing the seal, plug, and coil restored smooth operation.
Another case involved a marine engine with intermittent misfires at high load. The injector for cylinder 9 had restricted flow, causing a lean misfire. After ultrasonic cleaning, cylinder 9 performed normally and P0309 cleared.
Symptoms of P0309 – Cylinder 9 Misfire Detected
- Rough idle: Noticeable shaking or uneven running.
- Loss of power: Hesitation or sluggish acceleration.
- Poor fuel economy: Misfires reduce combustion efficiency.
- Check engine light: Flashing or steady depending on severity.
- Fuel smell: Unburned fuel exiting through exhaust.
- Engine stalling: Severe misfires can interrupt combustion stability.
Common Causes of P0309
Most Common Causes
- Worn or fouled spark plug in cylinder 9
- Faulty ignition coil or damaged coil boot
- Restricted or leaking fuel injector for cylinder 9
- Vacuum leak affecting cylinder 9
- Failing mass airflow (MAF) sensor causing incorrect air metering
Less Common Causes
- Low compression due to valve or piston wear
- Camshaft or crankshaft position sensor malfunction
- Weak fuel pump or failing pressure regulator
- ECU driver failure (rare)
- Carbon buildup affecting cylinder 9
Diagnosis: Step-By-Step Guide
Diagnosing P0309 requires careful inspection of the ignition system, fuel delivery, intake system, and mechanical condition of cylinder 9.
Tools You’ll Need
OBD-II scanner with misfire counters, spark tester, multimeter, compression tester, fuel pressure gauge, smoke machine, and basic hand tools.
- Check freeze-frame data: Note RPM, load, and temperature when the misfire occurred.
- Inspect spark plug: Look for wear, cracks, carbon buildup, or oil contamination.
- Swap ignition coil: Move coil 9 to another cylinder; if the misfire follows, the coil is faulty.
- Test injector performance: Use an injector balance test or swap injectors.
- Check for vacuum leaks: Smoke-test the intake runner for cylinder 9.
- Clean or test MAF sensor: Incorrect airflow readings may cause lean misfires.
- Review fuel trims: Lean trims suggest air leaks; rich trims may indicate injector leakage.
- Measure fuel pressure: Confirm pump and regulator output is within spec.
- Perform compression test: Low compression confirms mechanical faults.
- Check cam/crank timing: Timing errors or sensor faults may cause intermittent misfires.
Pro Tip: Cylinders located at the rear of multi-bank engines often run hotter and are prone to ignition component wear. Always inspect coil boots and plug seals closely.
Possible Fixes & Repair Costs
- Replace spark plug (cylinder 9): €20–€60
- Replace ignition coil: €50–€120
- Clean or replace fuel injector: €120–€260
- Repair vacuum leak: €50–€150
- Clean or replace MAF sensor: €20–€150
- Repair mechanical engine faults: €300–€900+
Can I Still Drive With P0309?
Driving with P0309 is not recommended. Unburned fuel from cylinder 9 can enter the catalytic converter and cause overheating or meltdown. If the check engine light is flashing, stop driving immediately to avoid severe engine or exhaust system damage.
Related Codes
- P0399 – Ignition Coil “R” Primary/Secondary Circuit
- P0364 – Reserved
- P0363 – Misfire Detected Fueling Disabled
- P0362 – Ignition Coil “L” Primary/Secondary Circuit
- P0361 – Ignition Coil “K” Primary/Secondary Circuit
- P0360 – Ignition Coil “J” Primary/Secondary Circuit
- P0359 – Ignition Coil “I” Primary/Secondary Circuit
- P0358 – Ignition Coil “H” Primary/Secondary Circuit
- P0357 – Ignition Coil “G” Primary/Secondary Circuit
- P0356 – Ignition Coil “F” Primary/Secondary Circuit
Key Takeaways
- P0309 indicates a misfire specifically in cylinder 9.
- Most common causes include spark plug, ignition coil, injector, or vacuum issues.
- A flashing check engine light means severe catalyst-damaging misfires.
- Early diagnosis keeps repair costs low and protects the engine.
FAQ
What is the most common cause of P0309?
A worn spark plug or failed ignition coil for cylinder 9 is the most frequent cause.
Can a vacuum leak trigger P0309?
Yes. A localized intake leak near cylinder 9 can create a lean misfire.
Is it safe to drive with a cylinder 9 misfire?
No. A misfire can damage the catalytic converter and reduce drivability.
How can I confirm a bad injector?
Perform an injector balance test or swap injectors to see if the misfire follows.
Should I replace all spark plugs at the same time?
If one plug is worn, replacing all plugs ensures consistent performance and prevents future misfires.