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Misfire

P0300: Random/Multiple Cylinder Misfire Detected

Title: SAE J2012 generic definition. Manufacturer-specific definitions and enhanced codes can differ; check service information for the exact vehicle.

What P0300 means

The engine computer counted misfires that are not limited to one cylinder, or that move around between cylinders.

What sets it

The ECM watches crankshaft speed. Each firing event should speed the crank up by a predictable amount; when a cylinder fails to contribute, the crank slows briefly and the ECM counts a misfire. If the count passes a calibrated threshold over a set number of revolutions and the misfires are spread across cylinders, it stores P0300. Cylinder-specific codes (P0301-P0312) often set alongside it.

Symptoms

  • Rough idle or shake, often worse under load or on hills
  • Hesitation or stumble on acceleration
  • Flashing check-engine light during heavy misfire
  • Fuel smell from the exhaust; possible catalyst code later

Is it safe to drive?

General guidance, not a guarantee: If the check-engine light is flashing, the misfire is severe enough to damage the catalytic converter. EPA advice is to minimize driving, avoid high speed and heavy loads, and get it repaired quickly. A steady light with a mild misfire is less urgent, but it should still be diagnosed soon. If in doubt, have it towed and diagnosed.

Common causes

Listed roughly from most to least common in general practice. The real order varies by vehicle, mileage and history.

  1. Lean condition from a vacuum or intake leak: an air leak affects every cylinder, so misfires show up randomly rather than on one cylinder
  2. Low fuel pressure or volume: a weak pump or restricted filter starves all cylinders, especially under load
  3. Worn spark plugs or several weak coils: plugs past their interval wear together, so the misfire spreads
  4. Contaminated or wrong fuel: water or very low-quality fuel can cause misfires on every cylinder
  5. Mechanical: low compression on several cylinders or cam timing off: worn timing chain, burnt valves or ring wear

Test before you replace: diagnostic sequence

General guidance only. Always follow the vehicle manufacturer's service information, specifications and safety procedures. Any figures below are general guidelines that defer to the manufacturer's specification.

  1. Confirm the complaint and record freeze frame: rpm, load, coolant temperature and fuel trims at the moment it set.
  2. Read misfire counters per cylinder (Mode $06 or the manufacturer misfire PIDs). "Random" codes often still lean toward 2-3 cylinders, which narrows the search.
  3. Check fuel trims at idle and at about 2,500 rpm. High positive trims that improve with rpm point to a vacuum leak; trims that worsen under load point to fuel delivery.
  4. Smoke-test the intake if trims are lean at idle.
  5. Test fuel pressure and volume against the manufacturer specification, ideally under load.
  6. Inspect plugs for wear, gap and fouling; inspect coils and boots for carbon tracking.
  7. If ignition and fuel check out, run a relative compression test with the scan tool or a scope, then a compression or leak-down test on the suspect cylinders.
  8. After repair, clear codes and confirm the misfire monitor runs clean under the same conditions as the freeze frame.

Bench note: Fix lean and fuel-delivery faults before buying plugs and coils. A random misfire with lean trims is usually an air or fuel problem, not an ignition problem.

How-to guides for this code

Related codes

P0301 Cylinder 1 Misfire DetectedP0302 Cylinder 2 Misfire DetectedP0303 Cylinder 3 Misfire DetectedP0304 Cylinder 4 Misfire DetectedP0171 System Too Lean (Bank 1)P0174 System Too Lean (Bank 2)P0420 Catalyst System Efficiency Below Threshold (Bank 1)P0430 Catalyst System Efficiency Below Threshold (Bank 2)P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

FAQ

Can a vacuum leak cause P0300?

Yes. Unmetered air leans out every cylinder, so the misfires spread around. That is why checking fuel trims is an early step.

Should I replace all the coils for P0300?

Not without testing. Coils tend to fail one at a time. If the plugs are worn, replace them as maintenance, but confirm the cause of a random misfire with fuel trims, fuel pressure and misfire counters first.

Why does the check-engine light flash?

On OBD II vehicles the light blinks while a misfire is happening that is severe enough to damage the catalyst, as required by California's OBD II rules. Reduce load and get it checked.

Sources

Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information. Shop Docs Studio is not affiliated with SAE, EPA, CARB or any vehicle manufacturer.

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