P0303 on a Honda
P0303 means cylinder 3 specifically is misfiring - the crank sensor sees that cylinder fail to contribute its share of rotation. Because the fault is isolated to one cylinder, Honda misfires are among the most satisfying codes to diagnose: the coil-swap test tells you the answer for free.
A flashing check-engine light changes the picture: that means raw fuel is reaching the catalytic converter. Ease off and get it diagnosed promptly - a melted converter turns a cheap fix into an expensive one.
- Ignition coil on cylinder 3: Coil-on-plug coils are the most common single-cylinder misfire cause on Civics, Accords and CR-Vs. The swap test below proves it without buying anything.
- Worn or fouled spark plug: If the plugs are at or past their interval (100,000 miles on most modern Hondas), replace the set - one fouled plug usually means the others are close behind.
- Clogged or weak fuel injector: Less common. If ignition checks out, injectors can be swapped between cylinders the same way coils can.
- Low compression in cylinder 3: The outcome nobody wants - valve or ring trouble. Only worth testing after ignition and fuel are ruled out.
Full P0303 guide for any vehicle