OBD-II Code P0306 — What It Means & How to Fix It

What Does This Error Mean?

P0306 means cylinder number 6 is misfiring. Your engine monitors each cylinder's contribution to rotation speed. When cylinder 6 fails to fire properly, the ECM detects the power loss and stores this code. You will likely feel rough running, engine shaking, and poor acceleration. The most common causes are a worn spark plug, bad ignition coil, or failing fuel injector.

Affected Models

6-cylinder, 8-cylinder, and higher vehicles only
Common in Ford Mustang GT V8
Common in Chevy Tahoe V8
Common in Toyota Camry V6
Common in BMW 3 Series 6-cyl

Common Causes

Worn or fouled spark plug in cylinder 6 no longer producing a reliable spark
Failed ignition coil on cylinder 6 unable to generate sufficient voltage
Clogged or leaking fuel injector in cylinder 6 causing improper fuel delivery
Cracked or worn coil boot allowing spark to leak before reaching the plug
Low cylinder compression from internal wear or a head gasket issue

How to Fix It

Perform the coil swap test first. Swap the cylinder 6 coil with the coil from cylinder 4 or another nearby cylinder. Clear the code and drive. If the misfire moves from cylinder 6 to the cylinder you swapped to, the coil is bad.

This zero-cost test confirms a bad coil before you spend money. It works on coil-on-plug (COP) systems, which most vehicles since the early 2000s use.
Inspect the spark plug in cylinder 6. Remove the coil, then unscrew the plug. Check for heavy carbon fouling (black), white chalky deposits (lean condition), or a severely worn center electrode. Replace the plug if it looks questionable.

On many V6 and V8 engines, the rear bank of cylinders (which often includes cylinder 6) can be harder to access. You may need to remove an intake manifold cover or intake tube. Take your time — do not force anything.
Check the spark plug gap on the new plug with a gap tool before installation. The correct gap is listed in your owner's manual or on a sticker under the hood. A plug with the wrong gap will cause an immediate misfire.

Most modern iridium or platinum plugs come pre-gapped but verify anyway. If adjusting, bend the side electrode gently — never touch the center electrode.
If plug and coil replacement do not fix the misfire, inspect the fuel injector for cylinder 6. Listen for a clicking sound from each injector with the engine running (a mechanic's stethoscope or long screwdriver pressed to the injector works). A silent injector is not firing.

A mechanic can use a noid light (a simple and cheap tool) to verify the ECM is sending the electrical signal to the injector. If the signal is there but the injector isn't clicking, the injector has failed.
If all ignition and fuel tests pass, do a compression test on cylinder 6. Crank the engine with the compression gauge installed. Compare cylinder 6 to the other cylinders. Low or zero compression means serious internal engine wear.

A wet compression test (add a small amount of oil through the plug hole and retest) can help distinguish between bad rings and bad valves. If compression rises with oil, the rings are worn.

When to Call a Professional

Try spark plug and coil replacement first — these are affordable DIY fixes. A single ignition coil costs $30-$100 and plugs cost $5-$20 each. If ignition parts don't fix it, a mechanic should test fuel injector flow and cylinder compression. Head gasket repair or internal engine work can cost $1,000-$4,000 — it requires professional diagnosis. Don't drive with a severe or flashing check engine light misfire — it damages the catalytic converter.

Frequently Asked Questions

Why does a misfire on cylinder 6 cause engine shaking?

Every cylinder fires in a specific sequence to keep the engine balanced. When cylinder 6 skips, one of those balancing pulses is missing. The engine rocks slightly with each missed firing event. This causes the vibration you feel in the seat, steering wheel, and throughout the cabin. The more cylinders misfiring, the more severe the shaking becomes.

The check engine light is flashing — is that worse than a steady light?

Yes, significantly worse. A flashing check engine light means a severe misfire is actively occurring. Unburned fuel is entering the catalytic converter and burning inside it. This can destroy the converter in minutes of driving. If the light is flashing, reduce speed, avoid hard acceleration, and get to a shop as soon as possible.

Does P0306 only occur in 6-cylinder engines?

No — it can occur in any engine with at least 6 cylinders. That includes 6-cylinder (V6 or inline-6), V8, V10, and V12 engines. If you have a 4-cylinder engine, there is no cylinder 6, so this code cannot appear. The cylinder numbering varies by manufacturer — cylinder 6 might be in different physical locations depending on the engine layout.