Guide step
Treat the timing of the misfire as evidence, not background noise
If the engine runs normally all week and then suddenly misfires within seconds of a fill-up, that timing is not a coincidence. An ignition misfire from a weak coil or worn plug usually does not wait for the exact moment after refueling to show up. A refuel misfire often means the intake just inhaled vapor it was never supposed to see, especially if the restart is rough only for thirty seconds to a few minutes and then mostly clears.
Guide step
What makes a refuel misfire feel different
Post-refuel EVAP flooding often feels rich and messy instead of cleanly dead. The engine may crank longer than usual, catch roughly, shake across multiple cylinders, smell strongly of fuel, puff brief black smoke, and sometimes flash the MIL because the catalyst is seeing raw fuel. That broad ugly pattern fits P0300 plus P0441, P0496, or even P0172 much better than one failed ignition coil on one cylinder.
Guide step
What makes an ignition misfire feel different
A true ignition misfire is less obsessed with the gas station. It may act up on cold start, under load, during rain, or anytime cylinder pressure rises. It often stays tied to one cylinder, one plug well, or one coil that can be moved and watched. If the misfire does not care whether the tank was just filled and there is no rich smell, no flooded restart, and no EVAP history, the ignition path deserves more attention.
Guide step
Use the code cluster to decide which branch gets first priority
P0300 by itself is only the headline. If it arrives with P0441 or P0496, push EVAP purge flooding much higher. If P0172 joins the party, that strengthens the case that extra vapor or fuel is being pulled into the engine. If the only companion codes are cylinder-specific P0301-P0304 with no refuel pattern, ignition or cylinder-specific fuel and compression faults move back to the front. If P0420 appears later, it may be the cost of driving an active misfire too long rather than the original cause.
Guide step
Fuel trim direction is one of the best tie-breakers
When EVAP vapor flooding is the root problem, fuel trims often swing negative around the rough restart because the ECU is trying to pull fuel away from a mixture that already went rich. When the misfire is caused by a vacuum leak, trims usually go positive instead. When it is a pure ignition failure, trim behavior can be noisy, but the timing clue and cylinder-specific pattern usually stay more useful than the raw trim number by itself.
Guide step
How to test the two stories without wasting parts
First, pull freeze-frame and ask whether the event happened right after a fill-up or hot soak. Second, inspect whether the purge valve can actually seal when closed, because a clicking solenoid that leaks is still a bad valve. Third, ask about topping off, canister saturation, repeated nozzle click-off, and fuel smell near the rear of the vehicle. Only after that should you start bulk-swapping coils or plugs, unless the data already points hard at one cylinder and one known ignition failure.
Guide step
Flashing MIL means urgency no matter which branch wins
Do not get so interested in the root cause that you ignore the risk level. A flashing check engine light means unburned fuel is reaching the catalyst now. Whether the source is a soaked canister feeding vapor through the purge path or a dead coil dumping one cylinder, the immediate rule is the same: reduce load, avoid extended driving, and diagnose it before the converter becomes the next bill.
Guide step
The practical bottom line
If the misfire appears right after refueling, acts rich, may clear as vapor burns off, and overlaps with P0441, P0496, P0172, or pressure-sensor clues, diagnose EVAP control before calling it a normal tune-up problem. If the misfire is repeatable regardless of fuel stop timing and follows a coil, plug, injector, or low-compression cylinder, treat it like an ordinary ignition or cylinder fault. The winner is the story that matches the timeline best.