Random/Multiple Cylinder Misfire Detected
The P0300 code means the vehicle's onboard diagnostic system has detected: Random/Multiple Cylinder Misfire Detected. This is a critical severity code.
- Keep driving?
- No -- stop driving
- DIY difficulty
- hard
- Estimated cost
- $50 for spark plugs up to $3,000+ for internal repair (e.g., a collapsed AFM/DFM lifter or cam/lifter failure)
The #1 fix for misfire codes. 15-piece set with 14mm, 16mm, 18mm magnetic sockets, T-handle extension, and spark plug gapper.
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Read and clear misfire codes. Shows which cylinder is misfiring so you know exactly what to replace.
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Common Symptoms
- Rough idle or engine shaking
- Loss of power during acceleration
- Check Engine Light flashing (catalytic converter damage risk)
- Poor fuel economy
- Engine may stumble or hesitate
Not sure this code matches what your car is doing? Try the guided checker: White, blue, or black smoke →
Probable Causes (Ranked by Likelihood)
- Worn or fouled spark plugs Very High
- Faulty ignition coils High
- Carbon-tracked coil boots arcing to the plug well (common on high-mileage Ford 4.6L/5.4L modular V8s) Moderate
- Clogged or failing fuel injectors High
- Low fuel pressure from weak pump Moderate
- Vacuum leaks at intake manifold Moderate
- Low compression in one or more cylinders Low
- Collapsed cylinder-deactivation lifter (GM 5.3L/6.2L AFM/DFM) or failed cam lobe/lifter (Dodge 5.7L HEMI MDS) -- uncommon but a major repair Low
- Faulty crankshaft position sensor Low
Step-by-Step Diagnostic Procedure
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Connect a scan tool and check for additional cylinder-specific codes (P0301-P0308). Start with the affected cylinder if one is indicated.
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Inspect spark plugs: remove and check for fouling, excessive gap (should be 0.028-0.060 in), cracks, or deposits. Replace if worn.
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Test ignition coils with a multimeter: primary resistance should be 0.4-2 ohms, secondary 6,000-15,000 ohms. Swap coils to see if the misfire follows.
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Check fuel injector resistance: most are 11-17 ohms. Listen for clicking with a mechanic's stethoscope at idle.
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Test fuel pressure at the rail: compare to spec (typically 45-65 psi). Check for pressure drop after key-off (leaking injector or failed regulator).
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Perform a relative compression test with a scan tool or cylinder balance test. Low compression below 120 psi warrants a wet/dry compression test.
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Check for vacuum leaks with carburetor cleaner or a smoke machine at all intake manifold gaskets, vacuum lines, and throttle body gasket.
Common Fixes by Vehicle
What techs usually find when diagnosing P0300 on specific vehicles — tap your vehicle for the fix and the exact part:
2007-2017 Toyota Camry 2.5L easy DIY
Replace all 4 ignition coils (Denso 673-1309). Toyota coils fail in pairs. Replace all at once with new spark plugs (NGK Iridium 91578). Clear codes and test drive for 20+ minutes to confirm. If misfire returns, check for intake manifold gasket vacuum leak.
Labor: 45 min2011-2020 Ford F-150 3.5L EcoBoost moderate DIY
Check for carbon buildup on intake valves (direct injection issue). If above 100K miles, walnut blast cleaning needed. Also check spark plugs -- Ford spec is replacement at 60K but they often foul earlier on EcoBoost. Use only Motorcraft SP-534 plugs gapped to 0.028 inch.
Labor: 1-2 hours (plugs), 3-4 hours (walnut blast)2008-2019 Honda Accord/Civic 2.4L easy (plugs), hard (valves) DIY
Start with spark plugs (NGK Iridium ILZKR7B11). Honda uses long-life iridium but they still wear out around 80-100K. If plugs are good, check valve adjustment -- Honda engines need periodic valve adjustments unlike most modern cars. Spec is 0.008-0.010 inch intake, 0.010-0.012 inch exhaust.
Labor: 30 min (plugs), 2 hours (valve adjust)2014-2020 Chevrolet Silverado/GMC Sierra 5.3L hard DIY
Active Fuel Management (AFM/DFM) lifter failure. Extremely common on GM 5.3L V8. Lifters collapse and cause misfires on cylinders 1, 4, 6, 7 (AFM cylinders). Fix requires lifter replacement or AFM delete kit. TSB #18-NA-355 addresses this. Also check for collapsed VLOM solenoids.
Labor: 6-8 hours2012-2020 Hyundai Sonata/Kia Optima 2.4L easy DIY
These Theta II engines have known connecting rod bearing failure and excessive oil consumption. Misfires can be caused by low oil level fouling plugs. Check oil level first. If burning oil, the engine may be covered under Hyundai/Kia engine settlement. Replace plugs with NGK SILZKR7B11 and monitor.
Labor: 30 minFrequently Asked Questions
Can I drive with a P0300 code?
If the Check Engine Light is flashing, stop driving immediately: a flashing CEL means the misfire is severe enough to damage the catalytic converter within minutes. A steady CEL is less urgent but the vehicle should be diagnosed within a day or two to prevent further damage.
What is the most common cause of P0300?
Worn spark plugs are the number one cause, especially on vehicles with 60,000 or more miles on the original plugs. This is the first thing to check and the least expensive fix.
Why does my P0300 only happen when the engine is cold?
Cold-start misfires often point to a failing coolant temperature sensor (causing a rich cold-start condition), a weak ignition coil that fails when cold, or sticky fuel injectors that do not atomize properly until warmed up.
Sources
This page is built from documented references. Verify against your own service info before repair work.
P0300 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
See All Codes & Fixes for Your Vehicle
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