OBD-II trouble code
P0306: Cylinder 6 Misfire Detected
A misfire isolated to cylinder 6 — the far-end cylinder on both inline-six and V6 engines. The part is cheap and the access usually is not, which changes what you should replace while the engine is open.
Quick facts
- System
- Powertrain
- Category
- Ignition and Misfire
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $75 – $1,600
- DIY difficulty
- Intermediate DIY
What does P0306 mean?
P0306 reports that cylinder number 6 is not producing its share of combustion. The engine control module (ECM) works this out by watching crankshaft acceleration between firing events, and because it can attribute the shortfall to one cylinder, you get a targeted code rather than the generic P0300.
Cylinder 6 is the easiest of the high-numbered misfires to locate, which is worth knowing because its neighbours are not. Whichever numbering convention a manufacturer uses, cylinder 6 ends up at the far end — the rearmost cylinder on an inline-six, and the last cylinder on the second bank of a V6. On a transverse V6 in a front-wheel-drive car, that means it sits against the firewall in the least accessible spot in the engine bay.
That location is not just an inconvenience, it is a cause. Rear-bank components on a transverse V6 sit in trapped, poorly ventilated air with radiant heat coming off the exhaust and no fan draft crossing them. Ignition coils are the parts that mind this most, and rear-bank coils commonly fail before their front-bank counterparts on the same vehicle at the same mileage. A P0306 on a high-mileage transverse V6 is a coil until proven otherwise.
The access problem should also reshape what you buy. When reaching cylinder 6 means removing an intake plenum, the labor dwarfs the parts, and replacing one plug while three are exposed and equally worn is a decision you will regret at the next misfire. On these engines the sensible move is to do all the rear-bank plugs — and often the rear-bank coils — in one visit. That reasoning does not apply to the front bank or to an inline engine where any plug is 20 minutes away.
If the check engine light is flashing, the misfire is severe and raw fuel is reaching the catalytic converter. Stop and have the car towed rather than driving it in. A steady light allows a careful drive to a shop with no hard acceleration.
Common causes
- Ignition coil failure on cylinder 6, disproportionately common on heat-trapped rear banks
- Worn or fouled spark plug in cylinder 6, often the last one anyone got around to changing
- Injector on cylinder 6 clogged, leaking, or electrically open
- Low compression from a burned valve, worn rings, or a head gasket failure at the end cylinder
- Cracked coil boot or a heat-hardened plug wire on older engines
- Vacuum leak at the end of the intake plenum feeding the cylinder 6 runner
- Carbon accumulation on the cylinder 6 intake valve on direct-injection engines
- Wiring to the rear-bank coil chafed or heat-damaged where it crosses the engine
Symptoms
- Check engine light on, or flashing while the misfire is active
- Shaking at idle that smooths out somewhat under load
- Loss of power and a flat spot on acceleration
- Unburned fuel smell from the exhaust
- Fuel economy dropping off
- Cranking longer than usual before the engine settles
- Emissions test failure regardless of how the car feels to drive
Diagnostic steps
- 1.Read the full code list before opening anything. P0306 alongside other misfires on the same bank points at a shared cause — fuel pressure, a plenum gasket, or the rear-bank coil supply — rather than one dead component.
- 2.On a transverse V6, treat the ignition coil as the leading suspect based on where cylinder 6 sits, and check its age and heat damage first.
- 3.Swap the cylinder 6 coil with one from the front bank, clear the codes, and drive. If the misfire follows the coil to the front bank, you have confirmed it without buying a part.
- 4.Inspect the coil boot and the connector for heat cracking, which is common on the rear bank even when the coil itself still works.
- 5.Pull the cylinder 6 plug and compare it with the others. Note that on a neglected rear bank all three may be equally worn, which is useful information for planning the repair.
- 6.Verify the injector clicks and check its resistance against the other five.
- 7.If the plenum has to come off to reach any of this, plan the job for the whole rear bank rather than one cylinder — the labor is already spent.
- 8.Compression-test cylinder 6 only after ignition and fuel come back clean, then leak-down test if compression is low.
Repair cost
$75 – $1,600
The parts are ordinary — $10 to $30 per plug, $40 to $200 per coil, $100 to $400 for an injector — but on a transverse V6 the intake plenum often has to come off to reach cylinder 6, adding one to two hours. That is why doing all three rear-bank plugs and coils at once frequently costs only slightly more than doing one, and why a quote for cylinder 6 work is usually higher than the same job on the front bank. Compression repairs run $1,000 and up.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with ignition coil replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.