P0306 Code: Cylinder 6 Misfire Detected
Quick Answer
Cylinder 6 is misfiring — combustion is failing in that specific cylinder.
At a Glance
What this code means
P0306 pinpoints cylinder 6 as the source of a misfire. On 6-cylinder engines, cylinder 6 is the last in the firing order. On V8 engines, its position varies. The causes are identical to other single-cylinder misfire codes — ignition, fuel, or mechanical failure within that cylinder.
A misfire happens when combustion fails in one cycle. The crankshaft loses speed at that cylinder, which the ECU detects as a misfire event.
Common causes
- Worn or fouled spark plug in cylinder 6
- Failed ignition coil for cylinder 6
- Leaking or clogged fuel injector on cylinder 6
- Low compression from worn rings or valves in cylinder 6
- Head gasket failure at cylinder 6 (on high-mileage engines)
Symptoms you might notice
- Rough running, especially at idle
- Steady or flashing check engine light
- Vibration at idle (more pronounced on inline 6-cylinder or certain V6 configurations)
- Reduced power on acceleration
- Possible black or unburned fuel smell from exhaust
Can you still drive?
Flashing check engine light — stop driving immediately. Steady light — short trips only until diagnosed.
How serious is this code?
High if misfire is severe. Address promptly to prevent catalytic converter damage.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Do the coil swap test before buying a coil — it's free and definitive
- If cylinder 6 shows low compression AND the adjacent cylinder (usually cylinder 5) also shows somewhat low compression, a head gasket or warped head could be causing a shared sealing failure
How to troubleshoot it
- Inspect the cylinder 6 spark plug for wear, fouling, or oil contamination
- Swap the ignition coil from cylinder 6 to another cylinder — if the misfire code follows the coil, it's the coil
- Test cylinder 6 compression — particularly compare to cylinder 5 (adjacent on most engines)
- Verify fuel injector operation with a noid light
- Check for head gasket issues if compression is very low and coolant level has been dropping
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- OBD-II scanner
- Compression tester
- Spark plug socket set
- Cooling system pressure tester
Estimated repair cost range
Spark plug: $5–$25. Ignition coil: $50–$150. Head gasket repair (if needed): $1,000–$2,500+.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
On some V-engines, cylinder 6 is located in a more confined space on the firewall side of the engine, making access to the spark plug and coil difficult. On those engines, even a simple spark plug replacement may require removing intake components.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.