OBD-II trouble code
P0300: Random/Multiple Cylinder Misfire Detected
The engine computer detected misfires across multiple cylinders or random cylinders it can't isolate. The check engine light may be flashing, the engine may run rough, and continued driving can damage the catalytic converter.
Quick facts
- System
- Powertrain
- Category
- Ignition and Misfire
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $50 – $1,500
- DIY difficulty
- Intermediate DIY
What does P0300 mean?
A misfire is a cylinder that fails to combust properly. The engine control module (ECM) detects misfires by monitoring crankshaft speed — a healthy combustion event accelerates the crankshaft slightly, and the ECM watches for the missing acceleration that signals a failed combustion event. When misfires occur across multiple cylinders or randomly across the engine, the ECM sets P0300 rather than a cylinder-specific code like P0301-P0306.
A flashing check engine light during P0300 indicates an active, high-rate misfire. Unburned fuel passes into the exhaust and overheats the catalytic converter, which can fail expensively if the misfire continues. Pull over safely if the CEL is flashing.
The most common cause of P0300 is worn or fouled spark plugs, especially on vehicles overdue for service. Ignition coil failures are the next most common, followed by fuel system, vacuum, and sensor issues. Internal mechanical problems — burned valves, broken piston rings, head gasket failures — are less common but possible on high-mileage or poorly-maintained engines.
Common causes
- Worn or fouled spark plugs (most common, especially past the recommended replacement interval)
- Failing ignition coils or coil packs
- Faulty fuel injectors (stuck open, clogged, or leaking)
- Low fuel pressure from a weak fuel pump or clogged filter
- Vacuum leak introducing unmetered air across multiple cylinders
- Bad fuel or water-contaminated fuel
- Failing camshaft or crankshaft position sensor
- EGR valve stuck open causing lean misfire
- Worn timing chain or jumped timing belt
- Internal engine damage (burned valve, low compression) — rare but possible
Symptoms
- Check engine light on or flashing
- Rough idle and engine shaking
- Loss of power on acceleration
- Poor fuel economy
- Hesitation or stumble at any speed
- Strong exhaust odor
- Hard starting or stalling
Diagnostic steps
- 1.Read freeze frame data — note RPM, load, temperature, and fuel trims when the code was set.
- 2.Pull any cylinder-specific misfire codes (P0301-P0306) that may be present alongside P0300.
- 3.Inspect spark plugs for wear, fouling, or wide gaps. Compare condition across cylinders.
- 4.Swap ignition coils between cylinders and rescan. If the misfire follows the swapped coil, the coil is bad.
- 5.Check fuel pressure against manufacturer specification at idle and under load.
- 6.Test for vacuum leaks using a smoke test or carb cleaner.
- 7.If basic ignition and fuel checks pass, perform a compression test to rule out mechanical damage.
Repair cost
$50 – $1,500
Spark plugs and labor for a tune-up run $50 to $300 on most vehicles. Ignition coil replacement adds $100 to $500 depending on how many failed. Fuel injector cleaning or replacement is $200 to $1,000. Internal engine repair starts at $1,500 and climbs quickly. Start with the cheap diagnostics first.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with spark plug replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.