Random / Multiple Cylinder Misfire Detected
In plain English: More than one cylinder is failing to burn its charge properly, and the computer can't pin it to a single one.
If the check engine light is flashing, stop driving and get it towed. A flashing light means fuel is going into the exhaust unburned, and that will destroy a catalytic converter in minutes, turning a two hundred dollar repair into a two thousand dollar one. If the light is steady, drive it gently and directly to where it will be fixed.
Most likely cause: worn spark plugs, coils or leads (38% of cases). Cheapest first test: is the check engine light flashing. Typical repair runs $10–$3000 in parts.
What P0300 actually means
The computer does not watch combustion directly. It watches the crankshaft. Every firing cylinder gives the crank a small kick, and the crankshaft position sensor measures the speed of every one. When a cylinder fails to fire, the crank slows very slightly on that stroke, and the computer counts it.
Those counts are kept per cylinder. If the misfires cluster on one cylinder you get a specific code, P zero three zero one through P zero three zero eight. When the counts are spread across several cylinders, or the computer cannot attribute them cleanly, it sets P zero three zero zero instead. That difference is the single most useful clue you have.
Symptoms
- Rough, shaking idleWorst at idle and often smooths out at higher RPM, which is why it can feel fine on the motorway.
- A flashing check engine light under loadThis is the urgent one. Flashing means active damage to the converter is happening right now.
- Hesitation or stumbling on accelerationUsually most obvious pulling away from a stop or climbing a hill.
- Noticeably worse fuel economyFuel that does not burn is fuel you paid for and threw out the tailpipe.
- A smell of raw fuel from the exhaustUnburned fuel passing straight through. Often accompanies the flashing light.
Most likely causes, ranked
Misfire counters — the reading that separates P0300 from P0301
Every scan tool worth owning can show misfire counters per cylinder. Warm the engine up, let it idle, and watch all of them at once for a minute.
A healthy engine sits flat at zero. Counters do not creep. If yours are flat at idle but the code keeps returning, you need to watch them under load instead, because some misfires only appear when the engine is working.
One cylinder climbing on its own is not really a random misfire at all. That is a cylinder specific fault, and it would normally set P zero three zero one through P zero three zero eight. Chase that one cylinder, and the job gets much cheaper.
Every counter climbing together is the true random misfire. That points at something all cylinders share: fuel pressure, an air leak, ignition timing, or the fuel itself. Not at one coil.
How to diagnose it, in order
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Is the check engine light flashing?Answer this before anything else. A flashing light is not a diagnostic step, it is a stop signal. Every minute you drive it is converter damage you will pay for later.Yes → Stop driving. Tow it.No → go to step 2
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Are there cylinder specific codes stored alongside it?Pull every code, not just the first one. If P zero three zero four is sitting next to P zero three zero zero, the computer has already localised it for you and you should work that cylinder first.Yes → Diagnose the named cylinder firstNo → go to step 3
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Are long term fuel trims well above ten percent?High positive trims mean the engine is running lean, and a lean mixture misfires everywhere at once. If trims are high, you are chasing an air leak or fuel delivery, not ignition parts.Yes → Diagnose the lean condition first — see P0171No → go to step 4
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Move a coil and plug to a different cylinder. Does the misfire follow the part?This is the best free test in the whole job. Swap the coil from a misfiring cylinder with a good one, clear the codes, and drive it. If the misfire moves with the part, you have found it without buying anything.Yes → Replace that coil or plugNo → go to step 5
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Is compression within spec and even across cylinders?Last, because it is the only step that costs real time. Look for a cylinder more than about ten percent below its neighbours, and follow up with a leak down test to find out whether it is rings, a valve or a gasket.Yes → Look again at fuel delivery and EGRNo → Mechanical repair — valves, rings or timing
Ignition parts first, then what every cylinder shares. Compression testing is last because it's the only step that costs real labour.
What you need to do this yourself
- A scan tool that reads live data — Every diagnosis on this site depends on watching sensor values in real time. A code-reader that only shows the code number cannot do it.
- Oxygen sensor socket — A slotted socket that clears the wiring pigtail. A normal socket will not fit.
- Digital multimeter — For checking sensor signals and grounds when live data alone isn't conclusive.
Some links may be affiliate links. They cost you nothing extra and do not change which tools are recommended — the diagnostic order on this site is the same either way.
What it costs to fix
| Repair | Parts | Labour |
|---|---|---|
| Spark plugs, full set Start here on any engine over sixty thousand miles on the same plugs. |
$25–$130 | 1.0 h |
| Ignition coil, single Many engines use one per cylinder, so you often replace only the failed one. |
$40–$280 | 0.5 h |
| Vacuum hose or intake gasket Cheap part, and the fix when high fuel trims pointed at a lean misfire. |
$10–$120 | 1.5 h |
| Fuel injector, single Try a cleaning service first on higher mileage engines. |
$60–$350 | 1.5 h |
| Compression or timing repair The branch everyone fears, and genuinely the least likely of the list. |
$400–$3000 | 8.0 h |
Roughly two thirds of these end at plugs and coils for under two hundred dollars. The swap test is what stops you spending the other numbers unnecessarily.