P0300: Random or Multiple Cylinder Misfire Detected
One rule before anything else: if the check engine light is flashing, stop driving. A flashing lamp is the computer's only way of saying the misfire is severe enough to destroy the catalytic converter right now, and a cooked cat turns a $300 problem into a $2,000 one in a single trip up the highway. A steady light with P0300 stored is less urgent, but this is still the code that punishes procrastination hardest.
What P0300 actually means
The computer tracks tiny variations in crankshaft speed. Every combustion event gives the crank a shove; a cylinder that fails to fire leaves a measurable stumble. When the stumbles come from multiple cylinders, or from no consistent one, it stores P0300: Random/Multiple Cylinder Misfire Detected. A misfire pinned to one cylinder logs its own code instead, P0301 through P0306, where the last digit is the cylinder number.
Can you keep driving?
Severity: 4 out of 5 flashing, 3 out of 5 steady. Flashing means pull over when safe and get the car looked at before driving further; every kilometre pumps raw fuel into a glowing converter. Steady means drive gently and diagnose within days, not weeks. This is also the wrong code to solve by clearing it and hoping.
What causes P0300 (most likely first)
- Worn spark plugs. The boring answer and the most common one. Plugs past their service interval misfire under load first, then everywhere.
- Failing ignition coils. Especially coil-on-plug designs, and especially in wet weather. A misfire that appears on damp mornings and fades once the engine bay dries out is a cracked coil boot introducing itself.
- A lean condition. Vacuum leaks starve cylinders unevenly. If P0171 sits alongside P0300, fix the lean cause and the misfires often vanish with it.
- Fuel delivery problems. Clogged injectors, a weak pump, or contaminated fuel. A misfire that started within a tank of servo fuel from an unfamiliar station is worth noting.
- Mechanical causes. Low compression from worn rings, a burnt valve or a slipped timing chain. Rarer, dearer, and usually accompanied by other symptoms.
Symptoms you'll notice
A rough, shaking idle. Hesitation or jerking under acceleration. Sometimes a fuel smell from the exhaust. The car may feel down on power, and economy drops because unburnt fuel is literally going out the pipe.
What it costs to fix in Australia
| Repair | Typical cost (AUD) |
|---|---|
| Spark plug set, fitted | $150–400 |
| Single ignition coil, fitted | $150–350 |
| Full coil set, fitted | $400–1,000 |
| Injector clean or replacement | $200–800 |
| Compression test | $120–250 |
The uncomfortable truth: if the plugs haven't been done in 80,000 km, replace them before paying for any diagnosis at all. It's due anyway, it's the most likely cause, and it resets the board for whatever testing follows.
P0300 on Australian cars
Falcon six coil packs are a running joke in Australian workshops for a reason, and VE Commodore coils are not far behind. Camry 2AZ engines misfire when oil consumption fouls the plugs. Hyundai i30 and Kia Cerato coils fail young often enough that many owners carry a spare. On any of these, a misfire chased with parts before plugs and coils are verified is money spent in the wrong order.
Will P0300 fail rego or a roadworthy?
Yes. The lit lamp fails a NSW pink slip and a Victorian or Queensland certificate at sale, and a rough-running engine can be defected on its own merits during a physical inspection. There is no version of P0300 worth carrying into an inspection.
How to read and clear P0300
Read all codes first, because a cylinder-specific code hiding behind P0300 hands you the diagnosis. Then use the misfire counters in live data to watch which cylinders are missing in real time. The classic confirmation test costs nothing: swap the suspect coil to another cylinder, clear the code, and see if the misfire follows the coil. The full library of codes is at our fault code index.
P0300 FAQ
What does a flashing check engine light mean?
An active, catalyst-damaging misfire. Stop driving as soon as it's safe. This is the one situation where the light itself is the emergency, not just a reminder.
Can bad fuel cause P0300?
Yes. Contaminated or stale fuel misfires across all cylinders at once, which is exactly the random pattern P0300 describes. If symptoms began right after a fill, run the tank down and refuel somewhere you trust before deeper diagnosis.
How is P0300 different from P0301?
P0300 means misfires across multiple or random cylinders. P0301 and its siblings name a single cylinder, which makes diagnosis much faster.
Can P0300 damage my car?
The misfire itself rarely hurts the engine quickly, but the raw fuel it sends downstream destroys catalytic converters, and that is routinely the most expensive consequence. See P0420 for where that road ends.
Will new spark plugs fix P0300?
Very often, yes, if they're due. If fresh plugs and healthy coils don't cure it, the search moves to fuel delivery and vacuum leaks.
Related codes: P0301 · P0171 · P0420 · Full fault code library
A fault code is a starting point, not a diagnosis. The same code can have different causes on different vehicles, and any costs shown are typical Australian ranges only. Always have the fault confirmed by a qualified mechanic before buying parts or booking repairs.