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OBD-II fault code definitions, symptoms, causes, and step-by-step diagnostics.
Standardized generic DTCIgnition & misfire

P0300 — Random/Multiple Cylinder Misfire Detected

The ECM detected misfire activity that is random, affects multiple cylinders, or cannot be assigned consistently to one cylinder.

Standardized generic definition
Random/Multiple Cylinder Misfire Detected
DTC category
Ignition & misfire
Diagnostic priority
High if the MIL is flashing or the engine is running roughly — continued driving can overheat the catalytic converter.
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What P0300 means

The ECM detected misfire activity that is random, affects multiple cylinders, or cannot be assigned consistently to one cylinder.

First priority: look at cylinder misfire counters and fuel trims before replacing ignition parts.

Possible symptoms

  • Flashing or steady Check Engine Light
  • Rough idle, hesitation, loss of power
  • Fuel odor or poor fuel economy
  • Possible hard starting

Causes to test

These are diagnostic possibilities, not a statistical parts-replacement ranking. Vehicle-specific service information takes priority.

  • Ignition problem: worn plug, damaged coil/boot, excessive plug gap, or wiring fault
  • Fuel delivery problem: injector fault, low fuel pressure, contaminated fuel, or injector imbalance
  • Unmetered air/vacuum leak or incorrect air measurement
  • Low compression, valve-train problem, timing issue, or other mechanical fault
  • On some vehicles, software/calibration or known service-bulletin condition

Step-by-step diagnostic workflow

  1. Do not clear codes; record freeze-frame and misfire counters first.
  2. If the MIL is flashing, avoid sustained driving and load.
  3. Identify whether the misfire is confined to one cylinder or affects multiple cylinders.
  4. For a single-cylinder misfire, inspect plug/coil/injector and compare that cylinder with a known-good cylinder where service procedures permit.
  5. Check fuel pressure, fuel trims, MAF/MAP data, and for intake leaks if multiple cylinders are affected.
  6. If ignition and fuel tests pass, perform compression/leak-down or valve-train/timing checks as appropriate.
  7. After repair, verify misfire counters remain stable during idle, cruise, and load.
Preserve evidence: EPA OBD guidance emphasizes that stored DTCs and freeze-frame data are important to proper diagnosis. Save them before clearing codes.

Scan data worth checking

  • Cylinder-specific misfire counters
  • Short- and long-term fuel trims
  • Engine load, RPM, coolant temperature, and spark advance
  • Fuel pressure or commanded rail pressure where supported

Severity and drivability

High if the MIL is flashing or the engine is running roughly — continued driving can overheat the catalytic converter.

Stop driving and investigate immediately if there is a flashing MIL, oil-pressure warning, overheating, fuel leakage, severe misfire, major loss of power, abnormal mechanical noise, or a transmission that is slipping badly.

Common diagnostic mistakes

  • Replacing all coils or injectors without isolating the affected cylinder
  • Continuing to drive with a flashing MIL
  • Condemning the catalytic converter before correcting an active misfire

Related generic codes

P0301 — Cylinder 1 Misfire DetectedP0302 — Cylinder 2 Misfire DetectedP0303 — Cylinder 3 Misfire DetectedP0304 — Cylinder 4 Misfire DetectedP0305 — Cylinder 5 Misfire DetectedP0306 — Cylinder 6 Misfire Detected

Source and accuracy notes

Generic DTC structure and standardized definitions are governed by SAE J2012/J2012DA. Actual enable criteria, thresholds, circuit pinouts, test values, software conditions, and repair procedures vary by vehicle. Use current OEM service information for the exact year/engine/calibration and check applicable manufacturer technical service bulletins. See the site methodology and primary-reference policy.