Ignition and misfire
P0300Random or multiple cylinder misfire detected
Random/Multiple Cylinder Misfire Detected
Stop driving or get it looked at now
Generic OBD-II code
Ignition/Combustion System (Misfire Monitor)
What this part does
The engine control module monitors crankshaft speed fluctuations to detect a cylinder that is not firing or burning fuel completely; the underlying cause is usually ignition, fuel delivery, or mechanical (compression/valve-train).
What the computer is measuring
The computer does not listen for a misfire. It watches how fast the crankshaft is turning, many times per revolution, using the crankshaft position sensor.
Every firing cylinder gives the crankshaft a small kick, and the sensor sees that as a brief acceleration. When a cylinder fails to fire properly, that kick is missing and the crankshaft slows very slightly for that fraction of a turn. The computer counts those missing kicks against the total, and once the rate crosses the threshold, it sets a misfire code.
If it can tie the missing kicks to the same cylinder each time, you get a cylinder specific code such as P0301. P0300 is what it sets when the misses are spread across several cylinders, or move around, or come too fast to attribute. That distinction is the single most useful piece of information the code carries: a random misfire points at something all the cylinders share, and a single cylinder misfire points at that cylinder’s own parts.
A flashing light rather than a steady one means the misfire is severe enough to be dumping raw fuel into the exhaust. That destroys a catalytic converter quickly, which is why a flashing light is a stop driving instruction and not a suggestion.
What you will notice
- A flashing check engine light while the misfire is happening. That flash means stop driving.
- The engine shakes or stumbles, worst at idle or under load.
- Hesitation or a loss of power when accelerating.
- Rough running that comes and goes with engine temperature or in the wet.
- A noticeable drop in fuel economy.
Likely causes, most common first
Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.
Worn spark plugs or failing ignition coils
The first thing to check on any misfire and the answer a good share of the time, especially past the service interval for plugs.
A vacuum leak
A cracked intake hose or a failed gasket leans out several cylinders at once, which is exactly the pattern that produces a random misfire rather than a single cylinder one.
Low fuel pressure or a failing fuel pump
Starves every cylinder together. Usually worse under load and when the tank is low.
A clogged or leaking set of fuel injectors
More often produces a single cylinder code, but a common cause such as bad fuel affects them all.
Loss of compression across multiple cylinders
A stretched timing chain, a slipped belt or worn valve train changes the timing on every cylinder at once. The least common of these, and the most expensive.
What to check, in order
Check whether any single cylinder codes are stored alongside it.
P0300 with P0302 means cylinder 2 is the worst of a wider problem, and gives you a place to start. P0300 completely alone points at something shared, namely air, fuel or timing.
Look at the fuel trim values at idle and at higher rpm.
Trims high at idle but normal at speed is the classic vacuum leak signature. Trims high everywhere points at fuel supply.
Inspect the intake for cracked hoses and listen for a hiss with the engine idling.
A leak found and fixed here often takes the misfire with it.
Pull the spark plugs and read them.
Plugs worn past their gap, oil fouled, or all noticeably different from each other each point somewhere specific. Uniformly clean plugs at correct gap move the search to fuel and compression.
Check fuel pressure against the specification, both at idle and while snapping the throttle.
Pressure that holds under load rules out the pump and the filter and leaves compression and timing.
Where this one turns up most
- Any engine at or past the spark plug replacement interval.
- Older vehicles where intake and vacuum hoses have gone hard and cracked.
Codes that often show up alongside it
What a DriveDecode adapter can read
A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.
Reading P0300 on your own car
DriveDecode pulls P0300 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.
Last reviewed 2026-09-05. Definition sourced from the published standard.