P0300
Fix within a few daysYour engine is experiencing misfires on multiple or random cylinders.
Code P0300 means your engine has random or multiple cylinder misfires. Common causes: Spark plugs/coils (40%), vacuum leaks (20%), fuel injectors (15%). Drivability: Limit driving, fix within a few days. Repair cost: $50-$300 DIY, $150-$1000 professional.
Can I Drive With P0300?
Limit driving, fix within a few days
Essential trips under 10 miles, avoid highways and heavy acceleration. Do not drive if the Check Engine Light is flashing.
Stop driving immediately if: Flashing Check Engine Light, loud engine noises, burning smell, rotten egg smell from exhaust, significant power loss or engine shaking.
Symptoms of P0300
- 1Check Engine Light is on (may flash during severe misfires)
- 2Engine runs rough or idles poorly
- 3Loss of engine power or hesitation during acceleration
- 4Reduced fuel economy
- 5Engine may shake or vibrate excessively
Common Causes of P0300
Parts most likely causing this code, ranked by probability
Worn spark plugs, failing ignition coils, or damaged spark plug wires prevent proper ignition of the fuel-air mixture.
40% of P0300 cases are fixed by replacing these ignition components
Unmetered air entering the engine through a cracked hose, gasket, or intake manifold causes an incorrect fuel-air mixture, leading to misfires.
20% of P0300 cases are fixed by repairing vacuum leaks
Clogged, dirty, or failing fuel injectors deliver an inconsistent or insufficient amount of fuel to the cylinders, causing misfires.
15% of P0300 cases are fixed by addressing fuel injector issues
A faulty MAF sensor sends incorrect air intake readings to the engine computer, leading to an improperly balanced fuel-air mixture and misfires.
10% of P0300 cases are fixed by replacing the MAF sensor
Insufficient fuel pressure from a failing fuel pump or regulator prevents proper fuel delivery to the injectors, causing misfires.
5% of P0300 cases are fixed by repairing fuel delivery issues
A stuck-open EGR valve allows exhaust gas into the intake at the wrong time, diluting the air-fuel mixture and causing misfires.
5% of P0300 cases are fixed by replacing the EGR valve
Failing O2 sensors provide inaccurate exhaust gas readings, causing the engine computer to adjust fuel trims incorrectly, which can lead to misfires.
3% of P0300 cases are fixed by replacing O2 sensors
Internal engine problems like worn piston rings, valve issues, or incorrect timing can prevent proper combustion, resulting in misfires.
2% of P0300 cases are fixed by addressing mechanical engine problems
Check These First (Before Buying Parts)
- Check for any other active or pending trouble codes with an OBD-II scanner.
- Visually inspect all vacuum hoses for cracks, disconnections, or signs of wear.
- Listen for any hissing sounds around the engine that might indicate a vacuum leak.
How to Diagnose P0300
- 1Connect an OBD-II scanner to check for additional codes (P0301-P0308, P0171/P0174, P0420/P0430).
- 2Visually inspect spark plugs, ignition coils, and spark plug wires for damage, cracks, or corrosion.
- 3Perform a vacuum leak test using a smoke machine or by carefully spraying carb cleaner around vacuum lines and intake manifold gaskets.
- 4Check fuel pressure at the fuel rail to ensure it meets manufacturer specifications.
- 5Test fuel injector resistance and spray pattern, or listen for their operation with a stethoscope.
- 6Monitor Mass Air Flow (MAF) sensor readings with a scan tool to ensure they are within specification.
- 7Check EGR valve operation; ensure it opens and closes correctly and is not stuck.
- 8Perform an engine compression test to rule out internal mechanical issues.
P0300 on Popular Vehicles
Honda Accord
2003-2024Honda Accords are prone to ignition coil failures and worn spark plugs, which are common causes for P0300. Sometimes, a valve adjustment is needed on higher mileage engines to resolve misfires.
60% fixed by ignition system components
Toyota Camry
2007-2024Toyota Camrys often experience P0300 due to failing ignition coils or worn spark plugs. Carbon buildup on intake valves (especially on direct-injection engines) can also contribute to misfires.
55% fixed by ignition system components
Ford F-150
2004-2024Ford F-150s, particularly those with 3-valve engines, are notorious for spark plug issues (breaking during removal) and ignition coil failures. Vacuum leaks, especially from the PCV system or intake manifold, are also common.
65% fixed by ignition system or vacuum leak repair
Frequently Asked Questions
What does P0300 mean?
P0300 indicates that your vehicle's engine computer has detected misfires occurring on multiple or random cylinders, meaning it cannot pinpoint the misfire to a single cylinder.
Can I drive with P0300?
Limit driving, fix within a few days. If the Check Engine Light is flashing, stop driving immediately. A flashing light means severe misfires are occurring, which can rapidly damage your catalytic converter.
Will P0300 clear itself?
No, P0300 will not clear itself. The underlying issue causing the misfires must be diagnosed and repaired. The code will remain stored until the problem is fixed and the system monitors complete their cycles, or it's manually cleared with a scan tool.
How do I fix P0300 myself?
Start by inspecting your spark plugs, ignition coils, and wires for wear or damage. Check for vacuum leaks around the intake manifold and vacuum hoses. If those look good, you may need to test fuel pressure or check fuel injector operation. A scan tool is essential for proper diagnosis.
How much does it cost to fix P0300?
DIY repair costs range from $50 for new spark plugs to $300 for ignition coils or a MAF sensor. Professional repair costs typically range from $150 for diagnosis and minor fixes to $1000 or more if multiple parts like fuel injectors or a catalytic converter are needed.
P0300 keeps coming back - why?
P0300 often returns because the root cause was not fully identified or addressed. Common reasons include overlooking a subtle vacuum leak, a failing fuel pump that only acts up under load, or an intermittent electrical issue with an ignition component or sensor.
Will P0300 fail emissions/smog test?
Yes, P0300 will cause your vehicle to fail an emissions or smog test. Misfires directly impact exhaust emissions, and the Check Engine Light being on (or having a pending code) is an automatic failure in most testing programs.
What's the difference between P0300 and P0301?
P0300 indicates a random or multiple cylinder misfire, meaning the engine computer can't isolate the problem to one cylinder. P0301 specifically points to a misfire on cylinder #1. P0300 suggests a more general issue affecting the entire engine, while P0301 points to a problem with components specific to that cylinder.
P0300 on Honda Accord
On Honda Accords, P0300 is frequently linked to failing ignition coils, especially on older models. Worn spark plugs are also a common culprit. Less often, a dirty or faulty EGR valve or a vacuum leak can trigger this code.
Questions to Ask Your Mechanic
Did you check for vacuum leaks using a smoke machine?
Good answer: Yes, we performed a smoke test and found no leaks, or we found and repaired a leak at [specific location].
What are the fuel pressure readings at idle and under load?
Good answer: Fuel pressure is [X] PSI at idle and [Y] PSI under load, which is within specification, or it's low/high, indicating a fuel delivery issue.
Have you checked the condition and operation of all ignition coils and spark plugs?
Good answer: Yes, we tested all coils and inspected the plugs. We found [specific issue] on [specific cylinder/component].
Repair Cost Estimate
DIY Repair
Parts only
$50-$300
Shop Repair
Parts + labor
$150-$1000
Save $100-$700 by doing it yourself
Difficulty
intermediate
Time
1-4 hours
Intermediate - requires a good scan tool for diagnosis and some mechanical aptitude to test various systems like fuel pressure and vacuum.
Cost of Waiting
Continued misfires send unburned fuel into your exhaust system, rapidly overheating and destroying your catalytic converter. A $50-$300 spark plug or coil job now prevents a $800-$2,500 catalytic converter replacement later.
While You're In There...
If you're replacing ignition coils, it's smart to replace spark plugs at the same time as they wear together and require the same labor to access.
These often degrade with heat and age. Replacing them with new coils or plugs ensures a good electrical connection and prevents future misfires.
A clogged PCV valve or cracked PCV hoses can cause vacuum leaks and contribute to misfires. It's a low-cost item often accessed during other engine work.
Tools You'll Need
- OBD-II Scan Tool
- Spark Plug Socket Set
- Torque Wrench
- Multimeter
- Vacuum Gauge
- Fuel Pressure Gauge (vehicle specific adapter may be needed)
- Smoke Machine (for vacuum leaks - optional, but highly recommended)
P0300 is one of the most common engine codes, indicating a general misfire. Most repairs are straightforward, often involving routine maintenance items, and your engine will run smoothly again afterward.
- Family-owned since 1957
- 990,000+ happy customers
- Fast nationwide shipping
- Easy returns
Your Next Steps
Step 1: Connect an OBD-II scanner to check for any additional codes (P0301-P0308, P0171/P0174) that can help narrow down the problem.
Step 2: Visually inspect your spark plugs, ignition coils, and spark plug wires for any obvious signs of damage, wear, or corrosion. Replace any suspect components.
Step 3: If ignition components look good, perform a thorough check for vacuum leaks using a smoke machine or by carefully listening for hissing sounds around the intake manifold and vacuum lines. Parts you'll likely need: set of 4-8 spark plugs, 1-8 ignition coils, vacuum hose kit.