What this code generally means, its most common causes, and how serious it is — before a real diagnostic for your specific car.
The engine control module has isolated the misfire to a single cylinder — number 3 — unlike P0300, which covers random or multi-cylinder misfires. The problem is therefore more likely tied to a part specific to that cylinder (its spark plug, coil, or injector) than a global engine issue.
As with any misfire, a steady light with an occasional misfire is usually still drivable short-term. A flashing light signals an active misfire — reduce speed and get the car checked without delay, to avoid damaging the catalytic converter.
Connect a Bluetooth OBD2 reader and read your vehicle's real codes with Meca — free, no appointment, with causes ranked for your actual situation.
🔌 Read my OBD2 codesA code that repeats on the same cylinder usually points to a part specific to that cylinder (its spark plug, coil, or injector) rather than a general engine issue — which is exactly what P0303 indicates.
P0300 means the control module sees random or spread-out misfires without isolating a cylinder. P0303 means it has specifically identified cylinder 3 as the source.
Some people do, to see if the problem "follows" the swapped part, but that's a hands-on mechanical step — Meca doesn't replace it, it's only meant to guide what comes next with a real code reader or a pro.