My 2016 sedan's check engine light came on yesterday, and the car is running a bit rough at idle but seems fine while driving. I borrowed a basic OBD2 scanner, and it's showing a P0302 code for a cylinder 2 misfire. I'm fairly handy and have changed spark plugs before, but I'm not sure if I should just start there or if this code could point to something more serious like a failing coil pack, fuel injector, or even a vacuum leak. What's the best diagnostic sequence to follow before throwing parts at it?
First, confirm the misfire with data instead of guessing. Re-scan and drive short but controlled, watching live data for cylinder 2 misfire events and real-time fuel trim. Check the freeze-frame data (RPM, load, temperature) to see under what conditions the misfire occurs. If it persists, move to the ignition path.
Check the cylinder 2 ignition hardware. Remove and inspect the spark plug for wear, gap, oil fouling, or heavy deposits. Inspect the spark plug boot and coil boot for cracks or resistance. If everything looks rough, swap the coil/coil boot from cylinder 2 to another cylinder to see if the misfire follows the coil. If it does, you’ve found the culprit; if not, you can rule out that coil as the issue.
Inspect the fuel side. Check the injector for cylinder 2: measure its resistance against spec and, if possible, swap injectors between cylinder 2 and another cylinder to see if the misfire follows the injector. Listen for injector click with a mechanic’s stethoscope and check for a clean spray pattern when you can. Also check for any fuel smell around the intake that could indicate a leaking injector.
Rule out vacuum and intake issues. With the engine idling, spray a little carb spray or brake cleaner around the intake gaskets, PCV line, and vacuum hoses and watch for a rise/fall in idle—an indication of a vacuum leak near cylinder 2. Check that the intake manifold gasket around that runner isn’t cracked.
Do a compression check on cylinder 2 and compare to the others. If compression is noticeably low, that points to a mechanical issue (valve seat, piston ring, head gasket) and will require more in-depth diagnostics. If compression is fine, you’re more likely dealing with ignition or fuel delivery.
If all of the above looks acceptable, consider a longer-term test plan: monitor the OBD data after repairs, clear codes, and see whether P0302 returns. If it does, you may want to consult a shop to do a professional cylinder balance test or check for a hidden engine issue like a marginal injector driver or a weak valve seal.