Learn how to diagnose P0300–P0306 misfires step-by-step—spark, fuel, air, and compression—using simple tools and real logic.
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What misfire codes really mean
- P0300 = random/multiple cylinder misfire
- P0301–P0306 = misfire detected on a specific cylinder
Misfires are not one problem—they’re a result. Your job is to find which category you’re in:
- Spark
- Fuel
- Air (vacuum / mixture)
- Compression / mechanical
- Sensor/ECU strategy issues (less common)
Step 1: Note when it misfires
This instantly narrows the cause:
- Idle-only misfire → vacuum leak, plug, coil, injector, compression
- Load/acceleration misfire → coil breakdown, plug gap, fuel delivery, boost leak
- Cold-start misfire → carbon buildup, injector spray pattern, weak ignition, compression
- Intermittent misfire → wiring, heat soak, marginal coil, moisture
Step 2: Start with the easiest wins (spark)
Spark issues are the most common DIY fix.
Check:
- Spark plugs (wear, oil fouling, wrong gap)
- Ignition coils (cracks, arcing, weak output)
- Coil connectors and harness tension
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Quick diagnostic move: swap test
If you have a cylinder-specific code (like P0302):
- Swap the coil to another cylinder
- Clear code, drive, recheck
- If misfire moves with the coil → coil is likely bad
Step 3: Confirm spark with a simple tester
A spark tester can quickly confirm whether ignition is weak on a cylinder.
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Step 4: Don’t ignore fuel (injectors + delivery)
Fuel-side misfires commonly come from:
- Dirty injector (flow imbalance)
- Injector electrical fault (open/short)
- Low fuel pressure under load
Clues it’s fuel:
- Misfire worse under acceleration
- Lean codes alongside misfire
- Misfire doesn’t follow coil/plugs
- Long crank or weak power at high RPM
Step 5: Compression matters (especially if one cylinder won’t cooperate)
If the misfire stays on one cylinder after spark checks, test compression.
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Compression red flags:
- One cylinder way lower than the rest
- Two adjacent cylinders low (possible head gasket)
- Low compression that improves with oil (ring wear)
Step 6: Mixture problems can look like “random misfire”
A vacuum leak can create random misfires—especially at idle.
If P0300 comes with lean codes (P0171/P0174), look at fuel trims and vacuum leaks.
Use AI to stop the parts cannon
Paste into WrenchWizardAI:
- Codes + cylinder number
- When it misfires (idle/load/cold)
- Fuel trims (if available)
- What you already swapped/tested
Hit Start Free Diagnosis and get a prioritized test list for your exact misfire—not generic advice.