Worn or fouled spark plug on cylinder 6; Failed ignition coil or coil-on-plug boot for cylinder 6; Faulty or clogged fuel injector for cylinder 6; Vacuum leak or intake manifold gasket leak affecting cylinder 6; Low compression from worn rings, a burnt valve, or a failed head gasket; Damaged wiring or connector to the cylinder 6 coil or injector; Mechanical valvetrain fault such as a worn cam lobe or collapsed lifter

DTC codes / P03xx · Ignition system and misfire / P0306

P0306Cylinder 6 Misfire Detected

Q: What does code P0306 mean?

Cylinder 6 Misfire Detected. P0306 means the engine computer has detected a misfire on cylinder 6, which is a failure of that cylinder to burn its air-fuel mixture properly during the power stroke. The computer watches crankshaft speed fluctuations and flags this code when the misfire rate exceeds a calibrated threshold that could damage the catalytic converter.

P0306 means the engine computer has detected a misfire on cylinder 6, which is a failure of that cylinder to burn its air-fuel mixture properly during the power stroke. The computer watches crankshaft speed fluctuations and flags this code when the misfire rate exceeds a calibrated threshold that could damage the catalytic converter.

SystemPowertrain

Code typeGeneric (all makes)

SeverityHigh

Fix difficultyIntermediate

AreaIgnition system and misfire

Symptoms you may notice

Often no symptoms besides the warning light, especially with a low-count misfire
Noticeable engine shake or vibration at idle that may smooth out at higher rpm
Rough running, hesitation, or a stumble during acceleration
A flashing check engine light when the misfire is severe enough to threaten the catalytic converter
Reduced power and poor fuel economy as the engine compensates
A raw fuel smell from the exhaust if unburned fuel is reaching the converter

Common causes, most likely first

Worn or fouled spark plug on cylinder 6
Failed ignition coil or coil-on-plug boot for cylinder 6
Faulty or clogged fuel injector for cylinder 6
Vacuum leak or intake manifold gasket leak affecting cylinder 6
Low compression from worn rings, a burnt valve, or a failed head gasket
Damaged wiring or connector to the cylinder 6 coil or injector
Mechanical valvetrain fault such as a worn cam lobe or collapsed lifter

How to diagnose and fix it

Scan all stored and pending codes and record freeze-frame data to confirm the misfire is isolated to cylinder 6
Inspect the cylinder 6 spark plug and ignition coil, swapping the coil to another cylinder to see if the misfire follows
Check the cylinder 6 injector connector and wiring, then test injector resistance and pulse with a noid light or scope
Perform a compression test and, if low, a cylinder leak-down test to rule out mechanical failure
Smoke-test the intake system for vacuum leaks that disproportionately affect cylinder 6
Replace the confirmed faulty component, clear the codes, and verify the misfire counter stays at zero during a road test
If the misfire persists with good spark, fuel, and compression, scope the ignition and injector signals to check the driver circuit

Why cylinder 6 misfires get misdiagnosed

A P0306 can be set by a failing catalytic converter or a restricted exhaust rather than by cylinder 6 itself, because backpressure can cause a single-cylinder misfire that moves around.
Swapping the coil and plug to another cylinder is the fastest way to prove whether the fault is ignition-related, yet many people replace parts on cylinder 6 without doing it.
A marginal fuel injector can pass a resistance test and still misfire only under load, so a scope or injector balance test is often needed.
Low compression on cylinder 6 from a worn cam lobe or valve seat can mimic an ignition fault and will not improve with new coils or plugs.
A flashing check engine light means the misfire is actively dumping fuel into the converter, and continuing to drive can destroy it within minutes.

Can I keep driving with P0306?

Do not keep driving if the check engine light is flashing, because unburned fuel will overheat and destroy the catalytic converter. With a steady light and mild symptoms, limit driving to a short trip to a repair shop and avoid heavy acceleration or towing.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.