P0300 — Random/Multiple Cylinder Misfire Detected means your engine’s ECU has detected that one or more cylinders are misfiring, but it can’t pinpoint which specific cylinder. It’s a serious code that can damage your catalytic converter if left unresolved. Here’s how to diagnose and fix it.
Quick Summary
| Code | P0300 |
| Description | Random/Multiple Cylinder Misfire Detected |
| Severity | High — can damage catalytic converter and O2 sensors |
| Common cause | Spark plugs, ignition coils, fuel injectors, vacuum leaks |
| DIY fixable? | Often yes — start with spark plugs and coils |
| Average repair cost | $100–$900 depending on cause |
P0300 vs P0301–P0308
If you see P0301, P0302, P0303 etc., those point to a specific cylinder (1, 2, 3). P0300 is the random/multiple misfire code — either multiple cylinders are misfiring, or the misfire is jumping between cylinders. Often all of these appear together.
Symptoms of P0300
- Check engine light — often flashing (a flashing CEL with P0300 means the misfire is severe enough to damage the cat — stop driving)
- Rough idle, shaking, or vibration
- Hesitation or stumbling on acceleration
- Poor fuel economy
- Engine feels like it’s “missing”
- Strong fuel smell from exhaust
Common Causes of P0300
- Worn spark plugs — The #1 cause on high-mileage vehicles. Spark plugs should be replaced every 30,000–60,000 miles (copper/platinum) or 100,000+ miles (iridium)
- Faulty ignition coils — Especially common on direct-ignition (coil-on-plug) engines. Coils can fail intermittently causing random misfires
- Bad fuel injectors — Clogged or leaking injectors cause lean/rich misfires in specific cylinders
- Vacuum leaks — Cracked hoses or intake manifold gaskets cause lean misfires that jump between cylinders
- Low fuel pressure — Failing fuel pump or clogged fuel filter starves cylinders
- Low compression — Worn piston rings, burned valves. Indicates serious engine wear
- EGR valve issues — Stuck-open EGR dilutes the air-fuel mixture causing misfires at idle
- Camshaft/crankshaft position sensor — Faulty sensors cause incorrect ignition timing
How to Diagnose P0300 — Step by Step
- Step 1: Check for companion codes — P0301–P0308 alongside P0300 confirms specific cylinders. Also check for lean codes (P0171/P0174) which point to vacuum leaks or fuel delivery issues
- Step 2: Inspect spark plugs — Pull and inspect. Black sooty deposits = rich mixture. White/grey = lean. Worn electrode = replace all plugs
- Step 3: Swap ignition coils — If you have coil-on-plug, swap the coil from the misfiring cylinder to an adjacent cylinder. If the misfire moves with the coil, the coil is bad
- Step 4: Check fuel injectors — With a mechanic’s stethoscope or screwdriver, listen to each injector clicking. Use live scanner data to check fuel trim values
- Step 5: Check for vacuum leaks — Spray carburetor cleaner carefully around intake manifold joints while the engine idles — a change in RPM indicates a leak
- Step 6: Compression test — If all else fails, do a compression test. Low compression on multiple cylinders indicates internal engine wear
P0300 Repair Cost by Cause
| Cause | Fix | DIY Cost | Workshop Cost |
|---|---|---|---|
| Spark plugs | Replace all plugs | $20–$80 | $100–$200 |
| Ignition coil(s) | Replace faulty coil(s) | $30–$80 each | $150–$300 |
| Fuel injector cleaning | Professional ultrasonic clean | $10–$20 additive | $50–$150 |
| Fuel injector replacement | Replace injector(s) | $50–$150 each | $200–$500 |
| Vacuum leak | Replace hose/gasket | $5–$50 | $100–$300 |
| Fuel pump | Replace pump | $100–$300 | $300–$800 |
⚠️ Important: Flashing Check Engine Light
If your check engine light is flashing (not steady) with a P0300 code, stop driving as soon as safely possible. A flashing CEL with a misfire means raw fuel is entering the catalytic converter — this can destroy a $1,000+ cat in minutes and cause a fire in extreme cases.
Related Codes
- P0301–P0308 — Specific cylinder misfires (Cylinder 1–8)
- P0171 / P0174 — System lean (Bank 1 / Bank 2) — often accompanies vacuum leak-related misfires
- P0420 — Cat efficiency fault (often caused by P0300 sending unburned fuel to the cat)
To diagnose this code accurately, you need a scanner with live data and misfire counter capability. See our Best OBD2 Scanners guide for recommendations at every budget.