1. Basic Checks (Do these FIRST):
* Visual Inspection: Look for anything obvious like a loose or damaged spark plug wire on cylinder 3, a cracked or damaged spark plug boot, or visible damage to the wiring harness near cylinder 3.
* Check the Spark Plug: Remove the spark plug from cylinder 3. Inspect it for damage (cracking, fouling, excessive wear). Gap should be correct (check your owner's manual or a reliable online source for the correct gap). If damaged, replace it.
* Check the Spark Plug Wire: Visually inspect the wire for cracks, fraying, or damage. If damaged, replace it. You can also use a spark tester to verify it's delivering a spark.
* Check the Ignition Coil: The 2003 Impala likely uses coil-on-plug (COP) ignition. Inspect the coil for cylinder 3 for damage. If possible, try swapping it with another coil (from a different cylinder) to see if the misfire moves. If the misfire moves, the coil is likely faulty.
* Check for Vacuum Leaks: A significant vacuum leak can cause misfires. Inspect all vacuum lines connected to the intake manifold for cracks or loose connections, especially near cylinder 3.
2. More Advanced Diagnostics (If the above don't solve the problem):
* Fuel Injector: A clogged or faulty fuel injector can prevent fuel from reaching cylinder 3. You can test the fuel injector's resistance (using a multimeter), but this requires some automotive electrical knowledge. A better option might be to use a fuel pressure gauge to measure fuel pressure. Low fuel pressure might suggest a fuel pump issue, fuel filter issue, or even a blockage in the fuel line.
* Compression Test: A low compression reading in cylinder 3 indicates a problem with the valves, piston rings, or head gasket. This requires a compression tester.
* Scanner/OBD-II Code Reader: Use a code reader to check for Diagnostic Trouble Codes (DTCs). These codes can pinpoint the problem more precisely. A P0303 code specifically indicates a cylinder 3 misfire. Other codes might also be present, providing additional clues.
* Professional Diagnosis: If you're uncomfortable performing these checks, take your car to a qualified mechanic. They have the tools and expertise to diagnose the problem accurately.
Important Considerations:
* Safety First: Disconnect the negative battery terminal before working on any ignition system components.
* Order of Operations: Always address the simpler, cheaper issues (spark plugs, wires, visual inspection) before moving on to more complex and expensive repairs.
* Professional Tools: Some diagnostic tests (e.g., compression test, fuel pressure test) require specialized tools.
By systematically checking these areas, you should be able to pinpoint the cause of the cylinder 3 misfire on your 2003 Chevy Impala. Remember to consult a repair manual or online resources specific to your vehicle for detailed procedures and specifications.
