P0300 Code: Random/Multiple Cylinder Misfire Detected
The engine computer sees misfires that jump between cylinders or hit several cylinders at once.
Stop driving
If the check-engine light is flashing, pull over as soon as it is safe and shut the engine off: unburned fuel is overheating the catalytic converter and can ruin it within a few miles or even start a fire under the car. With a steady light and only a mild stumble, drive gently to a shop and avoid hard acceleration, towing and highway speeds.
Safety first · Consult a professional
- Fire risk: stop driving if you smell burning or see smoke.
At a glance
Severity: High
- Meaning
- Random/Multiple Cylinder Misfire Detected
- System
- Ignition & Misfire · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $60–$2,500
- DIY difficulty
- Moderate
Your engine computer tracks crankshaft speed through the crankshaft position sensor, and every cylinder that fires gives the crank a small push. When a cylinder fails to burn its charge, that push is missing and the crank slows for a split second, which the computer counts as a misfire. P0300 means those misses are not tied to one cylinder: they wander around or show up on several cylinders at once. That usually points to something shared by the whole engine, such as worn plugs across the board, a vacuum leak or low fuel pressure, rather than one bad coil. When the misfire rate is high enough to overheat the catalytic converter, the check-engine light flashes instead of staying steady.
How to read P0300
- P Powertrain
- 0 Generic (SAE)
- 3 Ignition / misfire
- 00 Specific fault
Symptoms
- Check-engine light flashing under load, or on steady
- Engine shakes or stumbles at idle, often worse with the A/C on
- Hesitation or bucking when accelerating, especially uphill or when towing
- Noticeable loss of power and lower gas mileage
- Raw-fuel or rotten-egg smell from the tailpipe
- Harder starting on cold or damp mornings
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Worn or fouled spark plugs past their service interval. Many are rated for about 100,000 miles, but oil-burning or turbocharged engines wear them out much sooner.
- Vacuum leak (cracked PCV hose, leaking intake gasket, split brake booster hose) leaning out every cylinder, worst at idle
- Low fuel pressure from a weak fuel pump, clogged filter or failing pressure regulator
- More than one weak ignition coil, or coil boots arcing to ground after water got into the plug wells
- Dirty or failing mass airflow (MAF) sensor under-reporting airflow, so the engine runs lean
- EGR valve stuck open, diluting the intake charge at idle and low speed
- Mechanical problem: jumped timing chain or belt, low compression, or a leaking head gasket
Diagnosis step by step
Read freeze-frame data and per-cylinder misfire counts
Write down rpm, engine load, coolant temperature and fuel trims from the moment the code set. Then open the misfire counters (Mode $06 or the misfire page on a better scanner). A P0300 often has one or two cylinders doing most of the missing, and knowing which ones narrows the search.
Tools: OBD-II scanner
Check fuel trims for a lean condition
With the engine fully warm, add short-term and long-term fuel trim. If the total is above about +10% at idle and drops toward zero at 2,500 rpm, suspect a vacuum leak. If it stays lean at all speeds, suspect fuel delivery or the MAF sensor, especially if P0171/P0174 are stored too.
Tools: OBD-II scanner
Smoke-test the intake
Push smoke into the intake with the engine off and watch the PCV hoses, intake gasket, throttle body and brake booster line. Smoke finds leaks you won't hear at idle, and it is safer than spraying flammable cleaner around a hot engine.
Tools: Smoke machine, Flashlight
Pull and compare the spark plugs
Lay all the plugs out in cylinder order. Look for rounded electrodes, a gap well beyond spec, oil-wet tips or heavy carbon. Replace them as a full set with the type and gap specified for your engine, and torque them to spec, because aluminum heads strip easily.
Tools: Spark plug socket, Torque wrench
Test fuel pressure
On port-injected engines, connect a gauge to the fuel rail test port and compare key-on and idle pressure with the spec (often in the 40–60 psi range). Snap the throttle to see whether the pressure holds. On direct-injection engines, read low-side and high-side pressure from scan data instead.
Tools: Fuel pressure gauge, OBD-II scanner
Check the MAF sensor and the EGR valve
At a warm idle, MAF readings on most gas engines are roughly 1 g/s per liter of displacement. A much lower reading suggests a contaminated sensor, which you can clean with MAF cleaner only. Also make sure the EGR valve is fully closed at idle, because a stuck-open valve makes the idle lope.
Tools: OBD-II scanner
Run a compression test if the misfire persists
If ignition, fuel and air all check out, test compression on every cylinder. Readings more than about 10–15% apart, or two low cylinders next to each other, point to valve, ring or head-gasket damage or to a timing chain that has jumped.
Tools: Compression tester
Where the parts are
- 1MAF sensor
- 2Turbocharger
- 3Throttle body
- 4Intake manifold
- 5Fuel injector
- 6EGR valve
- 7Fuel pump
- 8Thermostat
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace spark plugs (full set; V6 rear-bank access costs more) | Yes | $120–$500 |
| Replace one or more ignition coils | Yes | $150–$450 |
| Repair vacuum leak (PCV hose, brake booster line or intake gasket) | No | $60–$500 |
| Replace fuel pump or fuel filter | No | $450–$1,100 |
| Replace catalytic converter damaged by misfire | No | $900–$2,500 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Ford
On F-150 pickups with the 3.5L EcoBoost, a stumble and misfire under hard acceleration in humid weather is a known pattern. Moisture condenses in the intercooler and gets swallowed all at once. Ford released updated parts and software for it, so ask a dealer to check whether your truck has them.
Honda
Honda V6 engines with Variable Cylinder Management (Accord, Odyssey, Pilot) can burn oil in the cylinders that shut off during cruising. That fouls their spark plugs and sets misfire codes. Oily, carbon-caked plugs on one bank are the telltale sign.
Field data
| Ford | 85 | ||
|---|---|---|---|
| Chevrolet | 49 | Traverse, Silverado 1500, Cruze | |
| Honda | 37 | ||
| Kia | 22 | Soul, Sedona, Sorento | |
| Hyundai | 19 | Tucson, Sonata, Elantra | |
| GMC | 15 | Sierra 1500, Terrain, Acadia | |
| Jeep | 11 | Grand Cherokee, Wrangler Unlimited, Cherokee | |
| Chrysler | 10 | 300, Town And Country, Pacifica |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
OBD-based smog checks fail any car with the check-engine light on, and a misfiring engine can also fail a tailpipe test on hydrocarbons. After the repair, drive a mix of city and highway miles until the readiness monitors are complete before you retest.
Frequently asked questions
Why does the check-engine light flash sometimes and stay steady other times?
A steady light means the computer saw enough misfires to raise emissions. A flashing light means the misfire rate is high enough to overheat the catalytic converter right now. The same fault can switch between the two depending on load, so a light that flashes going uphill needs attention immediately.
Will a new set of spark plugs fix P0300?
Often, if the plugs are overdue, since worn plugs affect all cylinders at once. If the plugs were recent, or the misfire only happens at idle, look at vacuum leaks, fuel pressure and the MAF sensor before spending money on coils.
Can bad gas cause random misfires?
Yes. Water in the fuel, or gas that has sat for months in a seldom-used vehicle, can cause misfires on several cylinders. If the misfire started right after a fill-up, running the tank low and refilling with fresh fuel from a busy station is a cheap first test.
Did the misfire already ruin my catalytic converter?
Not necessarily. Short periods of misfire usually do no lasting damage, but long drives with a flashing light can melt the substrate. If P0420 appears after the misfire is fixed, or you hear rattling from the converter, have it checked.
Why does P0300 only show up on rough roads?
Pothole jolts and driveline shudder briefly change crank speed and can look like misfires. Most vehicles pause misfire monitoring on rough pavement, but a damaged crank tone ring, a worn motor mount or a faulty wheel-speed signal can still let a false P0300 through.