P0303 on a GMC: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A steady light usually means you can drive a short distance, but a flashing light means the misfire is severe — pull over and get the truck or SUV towed.

What P0303 means on a GMC

On a GMC, P0303 means the engine computer found that cylinder 3 is misfiring — it is not burning fuel on schedule. Because the code points at one cylinder, the fault is almost always that cylinder's own spark plug, ignition coil, or fuel injector. GMC trucks and SUVs use coil-on-plug ignition, so each cylinder has its own coil.

Is P0303 common on GMCs?

P0303 shows up on GMC V8 trucks and SUVs like the Sierra and Yukon, and also on the Terrain and Acadia. Failing ignition coils and worn spark plugs are the usual causes, especially on high-mileage engines. A clogged injector is less common but possible.

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

Symptoms

Check engine light on, flashing if the misfire is severe
Rough idle or shaking while stopped
Loss of power or hesitation when accelerating
Worse fuel economy

Common causes

most common: Worn or fouled spark plug in cylinder 3
common: Failing ignition coil for cylinder 3
less common: Clogged or leaking fuel injector on cylinder 3
less common: Low compression in cylinder 3 from engine wear

How to fix it

Fix	DIY?	Typical cost
Replace the spark plugs	Yes	$60–180
Replace the cylinder 3 ignition coil	Yes	$70–220
Swap the coil with another cylinder to confirm the fault first	Yes	$10–30
Replace the fuel injector on cylinder 3	Mechanic	$250–650