P0300 Code: Random or Multiple Cylinder Misfire Detected

What happens if you don't fix it

Symptoms you can actually notice

• Noticeable shaking at idle
• Hesitation under acceleration
• Flashing check engine light
• Rough, uneven exhaust note

What the computer is actually complaining about

The crankshaft sensor watches the crank speed up slightly after each cylinder fires. When one cylinder fails to burn its charge, the crank slows at that exact moment. The computer counts those stumbles against each cylinder. Random misfires across several cylinders set P0300 instead of a single-cylinder code.

Most likely causes, cheapest check first

What you need in your hands

• OBD-II scanner with misfire counters

  Any $25 Bluetooth reader plus a free app shows per-cylinder misfire counts on most 2010 and newer cars.

• Spark plug socket and gap gauge Optional

  A 5/8 inch socket runs $8 and a gap gauge $4. Both fit any driveway toolbox.

• Spark tester Optional

  $15 at any parts store. Clamps onto a plug wire and shows the spark jumping in open air.

Test it yourself, step by step

Do these in order. Each step tells you what to do, what to look at, what counts as fine, what counts as broken, and what you will actually see.

1. 1 Plug the scanner into the trapezoid port under the dash on the driver side, above your left knee. With the engine idling, open the misfire counter screen.

   Look at

   Which cylinder numbers are counting up, and how fast they climb while the engine sits at idle.

   Fine if

   Every counter holds at zero and stays there for two full minutes.

   Broken if

   One or more counters climb steadily. Note the numbers, because a cylinder that misfires at idle points at ignition and one that only misfires under load points at fuel or compression.

   What you'll see A grid of cylinder numbers, most reading zero. The guilty one ticks upward every few seconds.

2. 2 Open the hood and look along the top of the engine for the coil packs, which sit in a row above the spark plugs on the valve cover.

   Look at

   Whether any coil looks burnt, cracked, or has a brown stain around its base. Look for a plug well full of oil.

   Fine if

   Every coil sits flush with a clean body and no oil pools around the boots.

   Broken if

   A coil with a melted or discolored body, or oil standing in a plug well. Oil in the well shorts the plug out.

   What you'll see Coils that all look alike. One that is darker than its neighbors is your first suspect.

3. 3 With the engine idling, listen along the intake tube that runs from the air box to the throttle body on the passenger side of the engine bay.

   Look at

   A hiss or whistle that changes when you wiggle the tube, and whether the engine stumbles when you spray a small amount of carb cleaner at a joint.

   Fine if

   A steady tone with no change when the tube is wiggled and no idle change when a joint is sprayed.

   Broken if

   The idle rises when you spray a joint, which proves unmetered air is getting in past the sensor.

   What you'll see A smooth hiss from the intake. A whistle that changes pitch as you move your hand means a split hose.

4. 4 Pull the spark plugs one at a time with the 5/8 inch socket, working from the front of the engine toward the rear. Lay them in a row on a bench in the order they came out.

   Look at

   Electrode wear, oil fouling, or a wet plug. Compare each plug against the one from the next cylinder.

   Fine if

   All plugs show a light tan color with a sharp, square electrode edge and a gap near the factory spec.

   Broken if

   One plug is black and wet, or its gap is worn wide open. A single odd plug points at that cylinder, not the whole engine.

   What you'll see Plugs that look almost identical. The odd one out tells you where to look next.

5. 5 If the misfire follows one cylinder, swap that coil with a neighbor along the top of the valve cover and drive 20 miles. Then reread the counters.

   Look at

   Whether the misfire counter moved to the cylinder that received the swapped coil.

   Fine if

   The misfire stays with the original cylinder, which clears the coil and points at the injector or compression.

   Broken if

   The misfire follows the coil to its new home. That coil is the fault and swapping it back confirms it.

   What you'll see The counter either stays put or moves. Either answer narrows the list by half for the cost of a drive.

What the repair costs

Reference range compiled from 3 source type(s), as of 2026-11. This is not a quote — actual cost depends on your vehicle, region, and which part turns out to be the real cause.

Basis: reference · 3 source types cross-checked · Data as of 2026-11 · Confidence: high

How to confirm it's really fixed

✓ Clear the code and drive two full warm-up cycles including one highway run of at least 20 minutes. If the misfire counters stay at zero through both cycles, the repair held. Any counter that starts climbing again means the cause was not fully addressed.

Common questions