P0300 — Random/Multiple Cylinder Misfire Detected
P0300 means combustion is failing in more than one cylinder. If the check engine light is flashing, stop driving: raw fuel is reaching the catalytic converter and will destroy it within minutes.
A steady light with a light misfire will usually get you to a workshop. A flashing light means the converter is being damaged right now, and continuing turns a cheap repair into an expensive one. This code will cause an emissions test failure.
What P0300 means
The engine control module watches the crankshaft for the tiny deceleration that follows a cylinder that failed to fire. When those events exceed a threshold within a set number of revolutions and cannot be attributed to one cylinder, it sets P0300 rather than a cylinder-specific code. Random misfire almost always points at something shared between cylinders — fuel pressure, a vacuum leak, timing, or compression — rather than at one coil or plug.
| System | Powertrain — ignition and misfire |
|---|
Symptoms you may notice
- Engine shakes or vibrates
- Check engine light flashing
- Hesitation or loss of power
- Rough or unstable idle
What causes P0300
Spark plugs worn out across the engine
Plugs wear at the same rate in every cylinder, so when they reach the end of their service life the misfire appears everywhere at once rather than in one place. The gap widens until the coil can no longer bridge it under the higher cylinder pressure of acceleration, which is why the fault often shows under load before it shows at idle.
Vacuum leak affecting several cylinders
A split intake boot, a failed manifold gasket or a disconnected hose admits air the airflow sensor never measured. Because the leak sits upstream of several cylinders, they all run lean together and misfire together.
Common misdiagnosis
Check fuel trims before replacing ignition parts. High positive trim at idle that normalises at speed is a vacuum leak, not a coil.
Fuel delivery cannot keep up
A tiring pump or a clogged filter holds pressure at idle but falls short under load. Every cylinder is starved at once, so the misfire is random by definition and typically worsens on hills or under acceleration.
Carbon build-up or lost compression
On high-mileage engines, carbon on the intake valves or worn rings reduce the cylinder's ability to burn cleanly. This tends to arrive gradually and affects several cylinders, and is the cause that remains when ignition, fuel and air have all been ruled out.
How to diagnose it
Check whether the light is flashing before anything else
Look at the malfunction indicator lamp with the engine running. A steady lamp and a flashing lamp are different faults with different urgency.
Safety
If the lamp is flashing, stop driving and have the vehicle recovered. Continuing destroys the catalytic converter, which typically costs several times the misfire repair.
Read every stored code and the misfire counters
Scan for all stored and pending codes, then read the per-cylinder misfire counters in live data. P0300 alongside a cylinder-specific code narrows the search immediately; counters spread evenly across cylinders confirm a shared cause.
Read short and long term fuel trim
With the engine warm, record both trim values at idle and at a steady 2500 rpm. The pattern separates an air problem from a fuel problem before any part is bought.
Inspect the spark plugs
Remove the plugs and compare them. Even wear across all cylinders with a gap beyond specification explains a random misfire on its own. Note any plug that is oil-fouled or noticeably different — that cylinder has a separate problem.
Safety
Let the engine cool completely. Removing plugs from a hot aluminium head can strip the threads.
Test fuel pressure under load
Fit a fuel pressure gauge and compare readings at idle and under a road load against the manufacturer specification. A pump that holds pressure at idle can still fall short when the engine demands fuel.
Safety
Fuel lines are pressurised and petrol vapour ignites easily. Depressurise the system and work away from any ignition source.
Related codes
Common questions
Can I drive with a P0300?
With a steady check engine light and a light misfire, usually far enough to reach a workshop. With a flashing light, no — raw fuel is reaching the catalytic converter and destroying it, which turns a cheap repair into an expensive one.
Why does P0300 appear instead of a cylinder-specific code?
The module sets P0300 when misfires are spread across cylinders rather than concentrated in one. That pattern points at something shared — fuel pressure, a vacuum leak, worn plugs across the set — rather than at a single coil or injector.
Will new spark plugs fix it?
Often, if the plugs are at the end of their service life and the fuel trims are normal. If trims are high, the cause is air or fuel and new plugs will change nothing.
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.