The P0302 code in your 2018 Honda Civic indicates a misfire in cylinder 2, and the vibration you're experiencing is the engine running unevenly from incomplete combustion. The 2018 Civic comes with either the 1.5L turbocharged or 2.0L naturally aspirated engine—both are 4-cylinder designs where a single misfiring cylinder creates noticeable imbalance.
Why Misfires Cause Vibration
A four-cylinder engine is inherently less balanced than a V6 or V8. When one cylinder doesn't fire properly, the imbalance becomes pronounced. You feel this as vibration, especially at idle or low speeds where the engine isn't producing enough power to mask the irregularity.
Cylinder 2 Specific Causes
Failed ignition coil for cylinder 2 is common—coils can fail individually. Worn or fouled spark plug in cylinder 2. Fuel injector problem preventing proper fuel delivery. Low compression from valve issues or head gasket leak. Vacuum leak near the cylinder 2 intake port.
1.5T Specific Considerations
The 1.5L turbo has been known for fuel dilution issues that can affect spark plug condition. If your oil smells like fuel, address the dilution issue alongside the misfire. Carbon buildup on direct injection valves can also contribute to misfires.
Diagnostic Approach
Using Honda HDS or a quality scanner, confirm cylinder 2 is the only affected cylinder. Swap the ignition coil from cylinder 2 to another cylinder—if the misfire follows, replace the coil. Check spark plug condition. Test fuel injector operation. On the 1.5T, check oil level and smell for fuel dilution.