Your 2021 Mazda3 vibrates noticeably at idle, and P0302 confirms cylinder 2 is misfiring. A single-cylinder misfire creates an imbalance that the four-cylinder engine shows plainly. Finding why cylinder 2 specifically is struggling leads to a targeted repair.
Why Single Cylinder Misfires Cause Vibration
The four-cylinder Skyactiv engine fires in a specific sequence designed for smooth operation. When one cylinder doesn't contribute its power stroke, the balanced forces become unbalanced. The result is vibration most noticeable at idle when there's no road movement to mask it.
Cylinder-Specific Causes
P0302 points specifically to cylinder 2, suggesting components unique to that cylinder have failed rather than engine-wide issues.
Spark plug failure on cylinder 2 is the most common cause. Plugs can crack, foul, or wear beyond function. The high-compression Skyactiv is sensitive to spark plug condition.
Ignition coil failure disables spark for that cylinder. Each cylinder has its own coil-on-plug; individual coils can fail from heat exposure, age, or manufacturing variation.
Fuel injector problems for cylinder 2—clogged, stuck, or electrically failed—prevent proper fuel delivery. The cylinder can't fire properly without correct fuel charge.
Compression loss from valve issues, piston ring wear, or head gasket breach on cylinder 2 reduces combustion pressure below what's needed for proper firing.
Carbon buildup specifically severe on cylinder 2's intake valve can cause that cylinder to misfire before others.
Diagnostic Process
Swap the ignition coil from cylinder 2 to another cylinder (cylinder 4 is easy). Clear codes and drive. If P0304 appears instead of P0302, the coil is bad.
If the coil tests good, swap the spark plug similarly. If the misfire follows the plug, replace it.
Test the fuel injector using a stethoscope (should click) or noid light (should pulse). Compare operation to other injectors.
Perform a compression test on cylinder 2 and compare to others. More than 10-15% variation indicates mechanical problems.
Skyactiv-Specific Notes
The 13:1 compression ratio makes ignition component quality critical. Always use Mazda-specified iridium spark plugs—cheaper plugs may not perform adequately in the high-compression chamber.
Carbon buildup on intake valves affects Skyactiv engines progressively. If other cylinders show similar tendencies, consider walnut blasting as part of the repair.
Repair Costs
Spark plug: $30-$60 installed (replace all four together: $100-$200 total).
Ignition coil: $80-$150 installed.
Fuel injector: $200-$400 installed.
If compression problems exist: $500+ depending on cause.
Carbon cleaning: $300-$500 if recommended.