P0303 GMC Terrain
Cylinder 3 Misfire Detected · Powertrain Code · High · Repair: Moderate
What is P0303?
Misfire detected in cylinder 3. Typically caused by a worn spark plug, bad ignition coil, faulty fuel injector, or low compression in that cylinder.
When the GMC Terrain triggers code P0303 (cylinder 3 misfire detected), it typically points to an issue in the powertrain system. This is a high severity code, so address it promptly to avoid further damage or safety concerns. Below you'll find the most common causes, symptoms, and repair options specific to the GMC Terrain.
This repair is rated moderate, meaning some experience and specialty tools required.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your GMC Terrain.
Common Causes on GMC Terrain
- Worn or fouled spark plug on cylinder 3
- Failed ignition coil on cylinder 3
- Fuel injector failure on cylinder 3
- Intake manifold gasket leak near cylinder 3 port
- Carbon buildup on intake valve (direct-injection GM engines)
Symptoms
- Check Engine Light on (may flash under load)
- Rough idle and vibration
- Hesitation and loss of power
- Increased fuel consumption
- Possible catalytic converter damage if ignored
Notes for GMC Terrain
On GM direct-injection engines like the 2.0T LSY, carbon buildup on intake valves can cause misfires — walnut blasting every 60k-80k miles is recommended preventive maintenance.