P0300: Random/Multiple Cylinder Misfire Detected

What this code means

Every time a cylinder fires, the crankshaft speeds up slightly. The engine computer watches the crankshaft speed sensor, and when a cylinder fails to fire it sees a tiny hesitation. Count enough of those and it sets a misfire code.

When the misfires all come from one cylinder you get a specific code like P0301 or P0304. P0300 is different: the misfires are moving around, or happening in several cylinders at once. That pattern usually points to something that affects the whole engine — air, fuel, or spark supply that every cylinder shares — rather than one bad part on one cylinder.

It's common to see P0300 together with single-cylinder codes (P0300 plus P0302, for example). In that case, start with the cylinder that has its own code, then recheck.

Symptoms you may notice

• Rough or shaky idle, especially at stoplights
• Hesitation, stumbling or jerking when you accelerate
• Check engine light that flashes under load
• Loss of power and worse fuel economy
• A fuel or rotten-egg smell from the exhaust

Common causes, most likely first

1. Worn spark plugs

   The most common cause on higher-mileage engines. Plugs wear evenly, so misfires show up on several cylinders instead of one.
2. Vacuum (intake air) leak

   A cracked hose, leaking intake gasket or a stuck-open PCV valve lets in air the computer didn't measure. The mixture goes lean and cylinders misfire at random, usually worst at idle.
3. Low fuel pressure or a weak fuel pump

   Every cylinder gets too little fuel. Often worse under hard acceleration or uphill. Clogged fuel filters do the same on cars that have a serviceable one.
4. Dirty or failing mass airflow (MAF) sensor

   If the sensor under-reports airflow, the computer adds too little fuel to every cylinder.
5. Bad or contaminated fuel

   Water or the wrong fuel after a fill-up can cause misfires across the board.
6. Ignition coil or wiring problem shared by several cylinders

   Less common on modern coil-on-plug engines, but a failing coil pack (on older designs) or a bad coil ground can affect more than one cylinder.
7. Mechanical engine wear

   Low compression from worn rings, a leaking head gasket or a jumped timing chain. Least likely, but worth ruling out if the simple causes check out.

What to check first

1. Note whether the check engine light is flashing or steady, and whether the misfire happens at idle, while cruising, or when accelerating. That alone narrows the cause a lot.
2. Look for other stored codes. Lean codes (P0171/P0174) alongside P0300 point to a vacuum leak, fuel delivery or the MAF sensor. Single-cylinder codes point to that cylinder first.
3. Check when the spark plugs were last replaced. Most modern plugs are rated for 60,000–100,000 miles; past that, they're the first suspect.
4. With the engine idling, listen for hissing around the intake and vacuum hoses. A shop can find small leaks quickly with a smoke test.
5. If the misfire is worse under load, have fuel pressure tested before replacing parts.
6. If the basics all check out, a compression or leak-down test tells you whether the engine itself is healthy.

Typical repairs and rough cost

Rough US shop prices for parts and labor. Your cost depends on the vehicle, the parts brand and local labor rates.

Official code definitions

“Generic” codes mean the same thing on every make. Manufacturer entries are that brand’s own description. Look up any other code in the code library.

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