Symptoms
- Check engine light flashing or on with a P0300-family code
- Engine shakes or feels rough, especially under load
- Possible occasional stumble or loss of power
- Reduced fuel economy
Likely causes
- CommonWorn spark plugs (multiple)Aged plugs across several cylinders cause scattered, random misfires.
- CommonVacuum leakExtra unmetered air leans out the mixture across multiple cylinders.
- PossibleBad gas or water in fuelContaminated fuel affects combustion broadly rather than one cylinder.
- PossibleLow fuel pressureInsufficient pressure can cause lean misfires especially under load.
- PossibleClogged fuel injectors (multiple)Deposits can affect several injectors similarly, especially with high-mileage or poor-quality fuel.
What to do
- A flashing check engine light means stop driving as soon as safely possible, since ongoing misfires can damage the catalytic converter.
- Get the exact code(s) read; P0300 is random/multiple, while P0301-P0308 point to a specific cylinder instead.
- Inspect spark plugs for wear, fouling, or the wrong gap if they're original or old.
- Check for vacuum leaks: hissing sounds, cracked hoses, or a loose intake boot.
- Consider whether the fuel is from a questionable source and try a fresh fill if so.
- Have fuel pressure and injector performance tested by a shop if plugs and vacuum checks don't resolve it.
Tools and parts
- OBD-II code reader
Spark plugsSearch: “spark plugs for [year make model]”
Ignition coilsSearch: “ignition coils for [year make model]”
When to call a pro
See a shop if the light is flashing, if misfires continue after basic checks, or if you're not comfortable inspecting ignition components yourself.
Safety
Use caution
A flashing check engine light during a misfire means stop driving soon; continued misfiring can send unburned fuel into the catalytic converter and cause expensive damage or overheating.