P0300
Random / Multiple Cylinder Misfire Detected
Published · Reviewed and updated
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.
A diagnostic starting point: Worn spark plugs, coils or leads. Start by asking: Is the check engine light flashing? Illustrative parts budgets span $10–$3000 USD, with diagnosis, labor and taxes extra.
Quick answers about P0300
- What does P0300 mean?
- More than one cylinder is failing to burn its charge properly, and the computer can't pin it to a single one.
- What should I check first for P0300?
- One starting point is Worn spark plugs, coils or leads. By a wide margin the most common. Plugs are a service item, and coils fail with age and heat. The code alone does not identify a failed part; follow the procedure for your vehicle.
- Can I drive with P0300?
- 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.
- How much does it cost to fix P0300?
- Illustrative parts budgets in this guide span $10–$3000 USD. Diagnosis, labor and taxes are extra. The cost depends on the demonstrated ignition, fuel, air or mechanical fault. Use test results to choose the repair; these parts budgets cannot price an untested misfire.
- How do I diagnose P0300?
- Start with: 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.
Watch the P0300 videos
Written correction, October 15, 2026: The written guide removes unsupported repair-frequency claims. The archived videos may still use earlier estimates; costs depend on the verified cause.
Full walkthrough — 8:38. Watch on YouTube
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, P0301 through P0308. When the counts are spread across several cylinders, or the computer cannot attribute them cleanly, it sets P0300 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.
Possible causes and what to check
This is a practical diagnostic order, not measured failure rates. Follow the tests and priorities specified for your vehicle; a code alone does not identify a failed part.
Misfire counters — the reading that separates P0300 from P0301
Illustrative graph, not a recording from your vehicle or a universal service limit. Confirm the test conditions, units and acceptance criteria in the applicable service procedure.
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 P0301 through P0308. 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
-
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
-
Are there cylinder specific codes stored alongside it?Pull every code, not just the first one. If P0304 is sitting next to P0300, 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
-
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
-
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
-
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 — Choose vehicle-compatible access to the live data, freeze frame and monitor results required by the diagnostic procedure. Some checks also need bidirectional control.
- Digital multimeter — Use the specified meter and test method for electrical checks. Follow circuit diagrams and avoid applying an unsuitable test load to a computer driver.
- Vehicle-specific service information — Confirm the procedure, safety precautions, tools and acceptance limits for the exact year, model and engine.
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.
Illustrative repair budgets
These are rough US planning examples, not quotes or measured repair averages. Parts and labor are separate; diagnosis, taxes, difficult access and additional repairs can increase the total.
| Repair | Parts (USD) | Labor time |
|---|---|---|
| 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 |
The cost depends on the demonstrated ignition, fuel, air or mechanical fault. Use test results to choose the repair; these parts budgets cannot price an untested misfire.
P0300 video summary · 1:01
A brief introduction to the code and a diagnostic starting point. Use the full guide and vehicle-specific service information for the complete procedure.
Written diagnostic walkthrough
Current written guidance organized by topic. This text may include corrections made after a video was published and is not a verbatim transcript of the archived video.
Meaning and driving guidance
Code P0300. 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.
How it is measured
Here's what your engine computer is actually measuring.
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, P0301 through P0308. When the counts are spread across several cylinders, or the computer cannot attribute them cleanly, it sets P0300 instead. That difference is the single most useful clue you have.
So the code is not a part failure report. It's a performance measurement that came back under spec.
Symptoms
What you'll usually notice, beyond the light itself.
Rough, shaking idle. Worst 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 load. This is the urgent one. Flashing means active damage to the converter is happening right now.
Hesitation or stumbling on acceleration. Usually most obvious pulling away from a stop or climbing a hill.
Noticeably worse fuel economy. Fuel that does not burn is fuel you paid for and threw out the tailpipe.
A smell of raw fuel from the exhaust. Unburned fuel passing straight through. Often accompanies the flashing light.
Possible causes
Here is a practical order for checking the possible causes. These are diagnostic priorities, not measured failure rates.
Check 1: Worn spark plugs, coils or leads. By a wide margin the most common. Plugs are a service item, and coils fail with age and heat.
Check 2: Unmetered air leak in the intake. A lean mixture misfires across every cylinder at once, which is exactly what random looks like.
Check 3: Weak fuel delivery — pump, filter or injectors. Low fuel pressure starves all cylinders together, so it presents as random rather than specific.
Check 4: Low compression, timing or valve problems. The expensive branch. A stretched timing chain or burnt valves misfire under load.
Check 5: EGR valve stuck open. Too much exhaust gas recirculated at idle dilutes the charge and it will not light.
Check 6: Contaminated or wrong fuel. Water or the wrong octane. Worth asking when the problem started relative to the last fill up.
Use the test results to choose the repair. A position in this list does not prove that a part has failed.
Illustrative diagnostic pattern
This illustrative graph explains a diagnostic pattern. Use the vehicle-specific procedure to interpret it.
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 P0301 through P0308. 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.
Diagnostic steps
So here is the order to actually test this, cheapest and most likely first.
Step 1. 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.
Step 2. Are there cylinder specific codes stored alongside it? Pull every code, not just the first one. If P0304 is sitting next to P0300, the computer has already localised it for you and you should work that cylinder first.
Step 3. 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.
Step 4. 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.
Step 5. 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.
Ignition parts first, then what every cylinder shares. Compression testing is last because it's the only step that costs real labour.
Illustrative repair budgets
These are illustrative US parts budgets with labor shown separately. Diagnosis, taxes, access and additional repairs can change the total.
Spark plugs, full set: 25 to 130 dollars in parts, plus about 1.0 hours of labor. Start here on any engine over sixty thousand miles on the same plugs.
Ignition coil, single: 40 to 280 dollars in parts, plus about 0.5 hours of labor. Many engines use one per cylinder, so you often replace only the failed one.
Vacuum hose or intake gasket: 10 to 120 dollars in parts, plus about 1.5 hours of labor. Cheap part, and the fix when high fuel trims pointed at a lean misfire.
Fuel injector, single: 60 to 350 dollars in parts, plus about 1.5 hours of labor. Try a cleaning service first on higher mileage engines.
Compression or timing repair: 400 to 3000 dollars in parts, plus about 8.0 hours of labor. The branch everyone fears, and genuinely the least likely of the list.
The cost depends on the demonstrated ignition, fuel, air or mechanical fault. Use test results to choose the repair; these parts budgets cannot price an untested misfire.
Sources and application limits
This legacy guide has not yet had a page-specific source list added. Check the factory service information for your exact year, make, model and engine before acting on a test or specification.