OBD codes / P0300
P0300
SeriousRandom/Multiple Cylinder Misfire Detected
Treat P0300 with some urgency. A steady check-engine light with a mild misfire is usually drivable short-term, but a FLASHING light means raw fuel is reaching the catalytic converter and can ruin it — pull over and have it towed or diagnosed. Misfires also tend to get worse, so do not put the fix off.
What this code means
A misfire means one or more cylinders are not burning their air-fuel mixture properly — the engine briefly loses power on that cylinder and the computer detects the hiccup in crankshaft speed. P0300 means the misfires are random or spread across several cylinders rather than isolated to one. The most common culprits are worn spark plugs or failing ignition coils, but a vacuum (air) leak, a fuel-delivery problem, low compression, or a sensor feeding bad data can all cause it. Because unburned fuel can reach and overheat the catalytic converter, a steady misfire should not be left for long.
Symptoms you might notice
- Rough idle or noticeable shaking, especially when stopped
- Loss of power, hesitation, or stumbling under acceleration
- Flashing check-engine light during an active misfire
- Reduced fuel economy
- Hard starting or an engine that surges
What it costs to fix
Typical range: $100–$1,000 · about ~0.5–3 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Spark plugs (full set) | $20–$120 | ~0.5–2 hrs |
| Ignition coil(s) | $30–$300 | ~0.5–1.5 hrs |
| Vacuum / intake leak repair | $20–$200 | ~0.5–2 hrs |
| Fuel injector | $100–$400 | ~1–3 hrs |
A plugs-and-coils fix is usually the low end; injectors, compression repairs, or a hard-to-reach coil on a V6/V8 push it higher. Labor climbs on engines where the intake or rear bank must come off to reach the parts.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0300 for my exact vehicle →
How to fix P0300
Moderate — doable at home with common tools and patience
- Pull codes and freeze-frame data
Read the code with a scanner and note the freeze-frame conditions (RPM, load, coolant temperature), plus per-cylinder misfire counters if your scanner shows them. Counts spread evenly across cylinders point at plugs, fuel, or a vacuum leak; counts clustered on one bank suggest a bank-specific problem like an intake gasket leak.
- Inspect the spark plugs
Pull two or three plugs and read them: rounded electrodes, heavy deposits, or plugs past their service interval mean a full set is the smart first repair. Fresh plugs (and replacing any cracked coil boots you find while you're in there) cure a large share of random-misfire cases.
- Check for vacuum leaks
With the engine idling, listen for hissing and carefully spray carb cleaner around intake manifold gaskets, vacuum hoses, and the brake booster line. If the idle changes when you hit a spot, you've found a leak that's leaning out the mixture and causing misfires, especially at idle.
- Look at fuel trims and fuel pressure
In live data, strongly positive long-term fuel trims mean the engine is running lean — a weak pump, clogged filter, or dirty injectors can starve multiple cylinders at once. A fuel pressure test against the spec for your engine confirms or rules out delivery problems.
- Test compression if the basics check out
A random misfire that survives fresh plugs, good coils, correct fuel pressure, and a leak-free intake calls for a compression or leak-down test. Low readings on one or more cylinders mean a mechanical problem (valves or rings) that needs a shop.
DIY or shop? Plugs, a visual coil-and-boot check, and the carb-cleaner vacuum test are all doable at home with basic hand tools, and they resolve most P0300s. Fuel-pressure and compression testing require gauges many DIYers don't own, and chasing an intermittent lean condition takes experience. If the misfire persists after fresh plugs and no obvious leaks, let a shop chase fuel delivery or compression rather than replacing parts on a guess.
Tools for the DIY steps: OBD-II scanner with live data · Spark plug socket and ratchet · Torque wrench · Carburetor cleaner (for the vacuum-leak test) · Spark plug gap gauge
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0300.
P0300 on specific vehicles
The likely causes shift by model. We’ve written up the patterns we see most:
Frequently asked
Can I drive with P0300?
With a steady (not flashing) check-engine light and only a mild misfire, short trips are usually fine — but get it looked at soon. If the light is flashing, an active misfire is dumping raw fuel into the catalytic converter and can destroy it; stop driving and have the car towed.
Why is my check-engine light flashing with P0300?
A flashing light signals a severe misfire happening right now. The danger is unburned fuel overheating the catalytic converter, an expensive part. Ease off the throttle, pull over when safe, and get it diagnosed before driving further.
What is the most common cause of P0300?
Worn spark plugs and failing ignition coils are the most common, especially if the plugs are overdue. Vacuum (air) leaks are the next most common. Fuel and compression problems are possible but less frequent, which is why a proper diagnosis saves money.
How much does it cost to fix P0300?
Often modest — a set of spark plugs or a single coil is commonly a low-hundreds repair. It climbs if the cause is fuel injectors, low compression, or parts that are hard to reach on a V6 or V8. Confirm the cause before replacing parts in bulk.
Will P0300 clear itself?
If it was a one-off from bad fuel or moisture the light may eventually go out, but a real misfire almost always returns until the cause is fixed. Do not ignore a repeating P0300 — misfires tend to worsen and can damage the converter.
Seeing P0300 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0300 for my vehicleSymptoms that trigger P0300
Drivers who get this code often notice one of these — each has its own likely-causes and cost guide:
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0300 from your phone — see our honest guide to the best OBD2 Bluetooth adapters.
Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.