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P0300 – Random/multiple cylinder misfire detected

What it means

The engine computer watches how the crankshaft turns. When a cylinder fails to fire properly, the crankshaft slows down very slightly at that moment. If the computer sees such misfires on several cylinders, or on changing ones, it sets P0300. If it can tie the misfire to one cylinder, it sets P0301–P0308 instead, where the last digit is the cylinder number. So P0300 tends to point to a shared cause that affects all cylinders.

Symptoms

• Rough, shaky idle; hesitation when accelerating.
• Loss of power, higher fuel use.
• A flashing check engine light with severe misfires.
• A fuel smell from the exhaust, sometimes popping.

Common causes (most likely first)

1. Worn spark plugs or failing ignition coils on several cylinders.
2. A vacuum leak or lean mixture (see P0171). This affects every cylinder at once.
3. Low fuel pressure, contaminated fuel or clogged injectors.
4. A MAF sensor fault that leads to a wrong mixture.
5. A mechanical cause: low compression, a valve problem or a timing chain or belt that has jumped.
6. Less often, interference or a wiring fault on the crankshaft sensor signal.

How urgent is it? Can I drive?

If the check engine light is flashing, do not keep driving under load. Unburnt fuel goes into the catalytic converter and can overheat it within minutes. That means expensive damage and even a fire risk. If the light is on steadily and the misfire is mild, you can drive gently for a short distance to a workshop.

What to check first

1. Read all codes. If a lean mixture (P0171/P0174) or MAF code is also present, deal with that first.
2. Look at the per-cylinder misfire counters in live data (if your scanner shows them). It may turn out that one or two cylinders are the main culprits.
3. Check the plugs and coils. A plug's condition (sooty, oily, worn) tells you a lot. If you move a coil to another cylinder and the misfire "moves" with it, you have found the faulty part.
4. Look at fuel trim. A large positive value points to a vacuum leak or a fuel shortage.
5. If all of that is fine, a compression test is next.

When to see a mechanic

Right away if the light is flashing. Also go if the plugs and coils are fine, because testing compression and timing and measuring fuel pressure are workshop jobs. On direct-injection engines, carbon build-up on the intake valves can also cause misfires. A mechanic can check this with a borescope.

FAQ

Will new spark plugs fix P0300?

Often, if the plugs are worn. If a vacuum leak or a fuel shortage is behind it, the code will come back.

Can bad fuel cause P0300?

Yes. Contaminated or watery fuel can cause misfires on several cylinders. It is especially suspicious if the problem started right after filling up.

What is the difference between P0300 and P0302?

P0302 names a specific cylinder (number 2). P0300 means the computer could not pin the misfire to one cylinder, or saw it on several.