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)
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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
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). Toyota coils fail in pairs. Replace all at once with new spark plugs (NGK Iridium plugs). Clear codes and test drive for 20+ minutes to confirm. If misfire returns, check for intake manifold gasket vacuum leak. Confirm the exact part for your year and engine with a retailer's fitment lookup or your VIN before ordering - these applications split by engine and build date.
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 plugs gapped to 0.028 inch. Confirm the exact part for your year and engine with a retailer's fitment lookup or your VIN before ordering - these applications split by engine and build date.
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 plugs). 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. Also check for collapsed VLOM solenoids. Your dealer can confirm whether an open service bulletin or warranty extension applies to your VIN. An AFM/DFM delete also needs an ECM tune, and because cylinder deactivation is part of the truck's emissions-certified configuration, deleting it can affect powertrain warranty coverage and road-legal emissions compliance - check your local rules before going that route.
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 and monitor. Confirm the exact part for your year and engine with a retailer's fitment lookup or your VIN before ordering - these applications split by engine and build date.
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 engine 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.
What actually makes the computer set a misfire code?
The OBD II regulation defines two separate misfire thresholds (Title 13 CCR 1968.2, section (e)(3.2)). One is misfire severe enough to damage the catalyst, evaluated in 200-revolution increments — and where the damaging rate would be under five percent, the manufacturer may simply set the limit at five percent. The other is misfire causing emissions to exceed 1.5 times the FTP standards. That is why a flashing light (catalyst-damage rate) means stop driving while a steady light for the same code does not. This is the threshold the monitor is designed around, not a description of your specific fault — the regulation says what the system must detect, not how to find it on your vehicle.
Mechanics Guide Step-by-step diagnosis, and what most sources get wrong
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 Misconceptions / Fact Check
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Commonly claimedA flashing check engine light is widely described as "the same warning, just more urgent."
VerifiedBlinking is a specific signal with its own rule, not an intensity setting. The OBD regulation requires the light to blink once per second at all times while misfire is actually occurring, because misfire at that level dumps unburnt fuel into the exhaust and can destroy the catalytic converter in minutes. Steady means a fault is stored. Blinking means damage is happening right now, and the correct response is to stop driving rather than to finish the trip.
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Commonly claimedIf the light stops flashing, most people read it as the problem having sorted itself out.
VerifiedThe regulation says otherwise, and the distinction is worth knowing. The light is only required to blink while misfire is occurring — it may go out entirely once the misfire pauses, and where another fault is also stored it simply drops back to a steady light. So a light that stopped flashing tells you the misfire is not happening at this moment, which is very often just a change in load or temperature. Nothing has been repaired, and the stored code and freeze frame are still there to be read.
Sources 2
This page is built from documented references. Verify against your own service info before repair work.
- California Air Resources Board, Title 13 CCR 1968.2 (2006 final regulation order)§(e)(3.4.1)(A)(ii) requires the MIL to blink once per second while misfire is occurring, and §(d)(2.3) sets the three-drive-cycle rule for extinguishing it
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
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:
Commonly Affected Vehicles
- Ford F-150 (4.6L, 5.4L)
- Chevrolet Silverado (5.3L)
- Toyota Camry (2.4L, 2.5L)
- Honda Accord (2.4L)
- Dodge Ram (4.7L, 5.7L HEMI)
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