P0306 on a GMC: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

If the check engine light is flashing, stop driving — a severe misfire can damage the catalytic converter. A steady light is usually safe for a short drive to a shop.

What P0306 means on a GMC

On a GMC, P0306 means the engine computer detected that cylinder 6 is misfiring, so that cylinder is not producing power on schedule. On GMC V6 models each cylinder has its own ignition coil and plug, so the fault usually points to cylinder 6's own parts. A flashing light means the misfire is severe enough to risk catalytic converter damage.

Is P0306 common on GMCs?

P0306 appears on GMC V6 models like the Acadia with the 3.6L engine and older Sierras with the 4.3L V6. Ignition coils and spark plugs are the usual causes as these engines rack up miles. Intake manifold gasket leaks and fuel injector problems can also set off a single-cylinder misfire on GM engines.

Commonly reported on: Acadia, Sierra, Terrain.

Symptoms

Check engine light on (flashing if severe)
Rough idle or shaking while idling
Loss of power or hesitation when accelerating
Poor fuel economy

Common causes

most common: Failing ignition coil on cylinder 6
common: Worn or fouled spark plug in cylinder 6
common: Clogged or leaking fuel injector on cylinder 6
less common: Leaking intake manifold gasket affecting cylinder 6
rare: Low compression in cylinder 6 from engine wear

How to fix it

Fix	DIY?	Typical cost
Replace the ignition coil on cylinder 6	Yes	$80–350
Replace worn spark plugs	Yes	$60–250
Replace the intake manifold gasket	Mechanic	$300–700
Run a compression test to rule out internal engine wear	Mechanic	$100–200