P0305 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 P0305 means on a GMC

On a GMC, P0305 means the engine computer detected a misfire on cylinder 5. That cylinder is not firing cleanly, which wastes fuel and can damage the catalytic converter if left alone. Because the code names one cylinder, the cause is usually its spark plug, ignition coil, or injector.

Is P0305 common on GMCs?

P0305 shows up on GMC V8 models like the Sierra and Yukon, and on V6 models like the Acadia. Worn spark plugs are the most common cause at higher mileage, and failing coil-on-plug ignition coils come next. Fuel injector issues are also known on GM V8 engines.

Commonly reported on: Sierra, Yukon, Acadia, Terrain.

Symptoms

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

Common causes

most common: Worn or fouled spark plug on cylinder 5
common: Failing ignition coil on cylinder 5
less common: Clogged or leaking fuel injector
less common: Wiring or connector issue at the coil
less common: Low compression from engine wear

How to fix it

Fix	DIY?	Typical cost
Replace the spark plug on cylinder 5 (or all plugs)	Yes	$40–150
Replace the ignition coil on cylinder 5	Yes	$80–250
Replace or clean the cylinder 5 injector	Mechanic	$200–500
Run a compression test to rule out engine wear	Mechanic	$100–200