P0300 — Random or Multiple Cylinder Misfire

Symptoms drivers report

• ▸The engine shakes or stumbles at idle, often smoothing out somewhat as engine speed rises.
• ▸Hesitation, surging or a flat spot under acceleration, worst going uphill or loaded.
• ▸A flashing check engine light while the misfire is happening — the severe case.
• ▸A smell of raw fuel at the tailpipe.
• ▸Fuel economy down sharply, commonly 15 to 30 percent.
• ▸Hard starting or an unusually long crank on a cold morning.

What triggers this code

1. 1

   Worn spark plugs or failing ignition coils

   Very common

   Plugs wear evenly, so they also fail evenly — which is exactly why worn plugs show up as a random misfire rather than a single-cylinder one. Gap opens up with mileage until the coil can no longer jump it reliably under load. Coils then run at higher voltage to compensate and start failing themselves, usually one after another over a few months.

   How to spot it

   The car has 80,000–100,000 miles on the original plugs, or the per-cylinder misfire counters in live data are spread fairly evenly across all cylinders.

   The fix

   Replace all spark plugs with the manufacturer-specified type and gap, and replace any coil with a cracked boot, carbon tracking or out-of-spec resistance.

   $150–$600•Moderate DIY
2. 2

   Vacuum leak in the intake system

   Very common

   Air entering downstream of the mass airflow sensor is air the computer does not know about, so it does not add fuel for it. The whole engine goes lean and cylinders start dropping combustion events at random. This is the reason P0300 and P0171 so often appear together, and when they do, the lean code is the one to chase.

   How to spot it

   Long-term fuel trim above +15% at idle that falls close to normal at 2,500 rpm, often with an audible hiss from the engine bay.

   The fix

   Smoke-test the intake and replace the cracked hose, split PCV line, brittle intake boot or leaking intake manifold gasket.

   $100–$500•Moderate DIY•p0171
3. 3

   Fuel delivery problem — weak pump, clogged filter or dirty injectors

   Common

   A pump that can hold pressure at idle but cannot keep up when the injectors open wider produces a misfire that only appears under load. Because all cylinders are fed by the same rail, all of them suffer at once, which is what makes it random rather than cylinder-specific.

   How to spot it

   The misfire shows up under load or at highway speed rather than at idle, and fuel pressure drops below spec when the throttle is opened.

   The fix

   Test rail pressure under load. Replace the fuel filter first if the vehicle has a serviceable one, then the pump or the injectors as the readings dictate.

   $200–$900•Advanced DIY
4. 4

   Low compression or a stretched timing chain

   Occasional

   Mechanical wear — worn rings, a burnt valve, a timing chain that has stretched enough to retard cam timing — takes the whole engine slightly out of tune. A stretched chain in particular produces a random misfire that is worst on a cold start, before the tensioner has full oil pressure.

   How to spot it

   A compression test shows cylinders below about 100 psi or more than 15% apart from each other; a stretched chain usually also sets a camshaft-to-crankshaft correlation code.

   The fix

   Compression and leak-down testing first, then timing chain replacement or engine repair depending on what the numbers show.

   $600–$4,000•Shop recommended
5. 5

   Bad fuel, water in the tank, or an EGR valve stuck open

   Occasional

   Contaminated or badly formulated fuel throws off combustion in every cylinder at once, and water in the tank does it dramatically. An exhaust gas recirculation valve stuck open has the same effect by design — it floods the intake with inert exhaust gas at idle when the engine cannot tolerate it.

   How to spot it

   The misfire started within a tank or two of a specific fill-up, or the idle is rough and unstable only at idle and clears completely above 1,500 rpm.

   The fix

   Drain and refill with fresh top-tier fuel, or clean or replace the EGR valve and its passages.

   $50–$600•Moderate DIY

How to diagnose it

1. 1

   Check whether the light is flashing before you do anything else

   A flashing check engine light changes the entire situation. It means the misfire rate is high enough that the computer considers converter damage imminent. Stop, shut the engine off and arrange a tow. Everything below can wait until the car is somewhere safe.

   Result: Separates a repair you can drive to from one you should not.

2. 2

   Look for cylinder-specific codes stored alongside P0300

   P0300 with P0302 and P0305 also stored is not a random misfire — it is two specific cylinders that also tripped the overall counter. Chase the specific codes and the general one usually goes with them. P0300 completely alone is the case that points at something shared by every cylinder.

   Tools: OBD-II scan tool

   Result: Narrows the search from the whole engine to one or two cylinders.

3. 3

   Read the freeze frame conditions

   Note the engine speed, calculated load, coolant temperature and fuel trim values recorded when the code set. A misfire at 700 rpm and low load with the engine cold is a very different fault from one at 3,000 rpm and 80% load. This tells you what driving conditions to reproduce when you go looking for it.

   Tools: OBD-II scan tool with freeze-frame support

   Result: Tells you whether to test at idle, under load, hot or cold.

4. 4

   Read the per-cylinder misfire counters in live data

   Most scan tools can show misfire counts per cylinder while the engine runs. Counts spread evenly across all cylinders point at fuel, air or timing. Counts concentrated on two cylinders that share a coil pack point at that coil. Counts on cylinders that are next to each other in the firing order often point at an intake manifold gasket leaking between two runners.

   Tools: Scan tool with misfire counter support

   Result: Turns a vague random misfire into a pattern you can act on.

5. 5

   Read the fuel trims at idle and at 2,500 rpm

   Long-term fuel trim above +15% at idle that improves at 2,500 rpm is the vacuum leak signature. Trims that are high at both speeds point at the mass airflow sensor or fuel delivery. Trims below −10% mean the engine is running rich, which suggests leaking injectors or excessive fuel pressure.

   Tools: OBD-II scan tool

   Result: Distinguishes an air problem from a fuel problem in about two minutes.

6. 6

   Pull the plugs and test pressure and compression

   Remove all the spark plugs and lay them out in cylinder order. Consistent light-brown tips mean ignition is likely fine. Oil-wet, black or heavily eroded plugs each tell a different story. If the plugs look acceptable, test fuel rail pressure under load and then run a compression test — those two cover the remaining causes.

   Tools: Spark plug socket, Fuel pressure gauge, Compression tester

   Result: Confirms or eliminates ignition, fuel and mechanical causes in order of cost.

Repairs & cost

Vehicles that set this code often

Design details that make some models more prone to this fault than others.

• Ford F-150

  The 5.4-liter three-valve V8 used from roughly 2004 to 2010 has two-piece spark plugs that seize in the aluminum head and break off at the 100,000-mile change interval. Many owners meet P0300 the first time the plugs are ever touched, and extraction of a broken plug is billed per cylinder.

• Chevrolet Silverado 1500

  V8s with Active Fuel Management shut down four cylinders in light cruise. A collapsed AFM lifter or the oil consumption these engines are known for fouls plugs on the deactivating cylinders, producing a misfire that comes and goes with driving conditions rather than a steady one.

• Hyundai Elantra

  Direct-injection four-cylinders build carbon on the intake valves because fuel never washes over them. The deposits disrupt airflow into several cylinders at once, which is why these show up as a random misfire rather than a single-cylinder code, and why walnut blasting fixes them when parts do not.

• Nissan Altima

  The QR25DE four-cylinder loses ignition coils one at a time. The code often starts as a single-cylinder misfire and becomes P0300 as a second coil weakens, which is why replacing the whole set is worth considering once the second one goes.

• 2021 Chevrolet EquinoxRandom misfires on this engine are frequently a downstream effect of the same PCV failure, because a large vacuum leak leans out all four cylinders at once. If the misfire is accompanied by P0171, fix the lean condition first rather than replacing coils.
• 2021 Chevrolet Silverado 1500On a 5.3L with Dynamic Fuel Management, a random misfire accompanied by a valvetrain tick is a collapsed lifter until proven otherwise. Do not let a shop replace plugs and coils repeatedly without a compression or cylinder-deactivation test.
• 2021 Ford EscapeOn a 1.5L Escape, treat a random misfire as a coolant intrusion question until proven otherwise. Check the coolant reservoir level and have a combustion leak test done on the coolant before spending money on coils or injectors — an ordinary ignition fault does not come with a slowly falling coolant level.
• 2021 Ford F-150Random misfire with no cylinder-specific code on an EcoBoost truck, appearing only in damp weather, usually means intercooler condensation rather than an ignition fault. On the 5.0L V8 with a cold-start rattle, the same code points at cam phaser wear.
• 2019 Ford F-150On the 5.0L V8, a random misfire alongside a cold-start rattle points at cam phaser wear affecting valve timing rather than ignition. Check whether the misfire is worst in the first minute after a cold start — that pattern is the phaser signature.
• 2021 Hyundai TucsonTreat a random misfire on the 2.4-liter GDI more seriously than on most engines. On this engine family it can be the first electronic sign of connecting rod bearing wear rather than a plug or coil issue, particularly if the car has also started consuming oil. Have the VIN checked for engine coverage before spending on ignition parts.
• 2021 Kia SportageRandom misfire on the 2.4L GDI is frequently carbon accumulation on the intake valves, since this engine has no port injectors to keep them clean. It shows up worst on cold starts and improves once warm — a pattern that helps separate it from an ignition failure.
• 2021 Mercedes-Benz C-ClassBecause the M264 is direct-injected with no port injectors, a random misfire that is worst on a cold start and improves as the engine warms points at intake valve carbon rather than ignition. Also check whether the 48V system has logged a separate fault — a belt starter-generator problem can present as rough running that looks like a misfire.
• 2021 Nissan AltimaThe 2.5L is direct-injected without port injectors, so intake valve carbon builds up over time and shows as a cold-start random misfire that clears as the engine warms. Check plugs and coils first, but if they are fine and the misfire pattern is cold-only, carbon is the likely answer.
• 2019 Toyota CamryA random misfire on a 2018–2019 Camry should push you to check the fuel pump recall status first. Fuel delivery problems from the recalled Denso pump present as random rather than cylinder-specific misfires, which is what separates this from a simple coil failure.
• 2021 Toyota CorollaRandom misfire on the 1.8L or 2.0L usually points at ignition coils or plugs approaching the 120,000-mile interval. Because both engines are chain-driven with no belt to slip, timing is rarely the culprit here — check coils by swapping one to a different cylinder and seeing if the misfire follows it.

Flashing versus steady: two different emergencies

Why random misfires are harder than single-cylinder ones

Frequently asked questions

Related information