OBD-II · P0300
P0300 Code: Random Multiple Cylinder Misfire — Causes & Fixes
Random / multiple cylinder misfire
The short version
Short careful drives are usually fine, but a flashing check-engine light means active misfiring that can quickly destroy the catalytic converter — don't put off diagnosis more tha…
- Severity: Serious
- Most common cause: Worn spark plugs or incorrect gap
- 6 diagnosis steps in order
- 5 symptoms tied to this code
Darker bars are the more common causes; bar length is the top expected repair cost. Start with the dark, cheap causes before the expensive ones.
Can I keep driving?
Short careful drives are usually fine, but a flashing check-engine light means active misfiring that can quickly destroy the catalytic converter — don't put off diagnosis more than a day or two.
Symptoms
- Noticeable engine shake at idle or under load
- Loss of power and hesitation on acceleration
- Noticeably higher fuel consumption
- Smell of unburnt fuel from the exhaust
- Check engine light flashing instead of staying solid
Causes & repair cost
Worn spark plugs or incorrect gap
Most common$40 – $160
Failed ignition coil(s)
Most common$120 – $400
Vacuum/intake manifold leak
Possible$150 – $450
Dirty fuel injectors or low fuel pressure
Possible$150 – $600
Blown head gasket (rare, more serious)
Rare$1,500 – $3,500
Cost depends heavily on how many plugs/coils fail and the vehicle's cylinder count; ranges vary widely between a 4-cylinder economy car and a V8 or turbo engine, so always get a shop quote after diagnosis.
Estimate the repair for your carDiagnosis steps in order
- Pull freeze-frame data and any cylinder-specific codes (P0301-P0308)
- Inspect spark plugs visually and check gap
- Swap a coil between cylinders and see if the misfire follows
- Check for vacuum leaks with smoke test or spray method
- Test fuel pressure with a gauge
- Run a compression test if the misfire persists
FAQ
Can I still drive with a P0300 code?
For short trips yes, but a flashing check-engine light means you should stop driving to avoid catalytic converter damage.
What is the most common cause of P0300?
Worn spark plugs and failing ignition coils are the most common culprits.
Does replacing spark plugs always fix P0300?
Often, if plugs were the root cause — but confirm with diagnosis before replacing parts blindly.
Can a vacuum leak cause P0300?
Yes, unmetered air entering the intake leans out the mixture and can trigger random misfires.
Related codes
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