When your 2018 Chevrolet Impala sets a P0303 code along with noticeable engine vibration, cylinder 3 isn't contributing its normal power pulse. The Impala's available engines—2.5L four-cylinder and 3.6L V6—each have different cylinder arrangements affecting diagnosis, but the systematic approach remains similar.
Understanding P0303
P0303 indicates the PCM has detected consistent misfires specifically in cylinder 3. The system monitors crankshaft acceleration—when cylinder 3 should fire but doesn't produce expected acceleration, it registers as a misfire. The vibration you feel results from the engine's imbalanced power delivery.
Cylinder 3 Location
On the 2.5L four-cylinder, cylinder 3 is on the rear half of the engine (closer to the firewall). On the 3.6L V6, cylinder 3 is typically the front cylinder on the rear bank. Knowing the exact location helps when accessing components for testing and repair.
Common Causes
Ignition coil failure for cylinder 3 is most common—coils fail individually on GM engines. Spark plug wear, damage, or incorrect gap prevents proper ignition. Fuel injector problems—clogged, stuck, or electrically failed—affect fuel delivery. Vacuum leaks at the cylinder 3 intake runner create lean conditions. Low compression from valve or piston issues causes mechanical misfire.
Diagnostic Process
Swap the cylinder 3 ignition coil with an adjacent cylinder. Clear codes and drive. If the misfire moves to the new cylinder, the coil is faulty—an easy fix. If the misfire stays at cylinder 3, swap the spark plug. Test the fuel injector with a stethoscope—it should click rhythmically. Compression test identifies mechanical issues.
Repair Costs
Ignition coil replacement costs $100-$200 per coil. Spark plug replacement for all cylinders runs $150-$300 depending on engine. Fuel injector replacement costs $200-$350 per injector. More extensive repairs for compression issues can exceed $1,000.