Code
P0300
GWM
P — Powertrain
- The order of ignition of cylinders is disrupted
AI status
Completed
Completed
100%
Causes
- Worn or fouled spark plugs
- Faulty ignition coils or high-tension leads
- Clogged, leaking, or failing fuel injectors
- Low fuel pressure or weak fuel pump
- Vacuum leaks or intake air leaks
- Incorrect air/fuel mixture (MAF/IAT issues)
Symptoms
- Rough idle and engine vibration
- Reduced engine power and poor acceleration
- Illuminated Check Engine Light (MIL)
- Intermittent stalling or difficulty starting
- Poor fuel economy
- Visible exhaust smoke (in some cases)
What to check
- Retrieve stored and pending codes and freeze-frame data with a scan tool
- View misfire counters/which cylinders are reporting misfires
- Monitor live data: RPM, fuel trims (STFT/LTFT), O2 sensor voltages, MAF/MAP, injector pulse widths
- Visual inspection: spark plugs, ignition coils, wires, fuel lines, vacuum hoses and intake boot
- Fuel pressure check at the rail (static and under load)
- Swap ignition coils between cylinders to see if misfire follows the coil
Signal parameters
- Misfire counts per cylinder (counts/min or event count)
- Engine RPM stability (fluctuations indicated during idle/drive)
- Short-term and long-term fuel trim (%)
- O2 sensor voltages and switching frequency (V)
- MAF mass airflow (g/s) or MAP vacuum readings (kPa/inHg)
- Fuel rail pressure (kPa or bar) and fuel pressure regulator behavior
Diagnostic algorithm
- Read and record all DTCs, freeze-frame and misfire counters with a capable scan tool.
- Verify symptom: test drive or idle observation to reproduce misfire and note affected cylinders (if any).
- Inspect ignition components: remove and inspect spark plugs for fouling/wear; check coil connections and swap coils between cylinders to see if misfire moves.
- Check fuel delivery: measure fuel rail pressure (static and under load). Test injector operation (noid light or scope) and consider swapping injectors to isolate.
- Monitor live data: review STFT/LTFT, O2 response, MAF/MAP and injector pulse widths to identify lean/rich conditions or an airflow/sensor fault.
- Test for intake/vacuum leaks (smoke test, spray test where safe) and inspect intake manifold boots and PCV system.
- Perform compression or leak-down test on cylinders to rule out mechanical failures (worn rings, valves, head gasket).
- Verify engine timing and cam/crank sensor signals (scope or scan tool) to ensure proper synchronization; inspect timing belt/chain condition and alignment marks.
- Inspect exhaust system for restriction, check catalytic converter temperature or backpressure if suspected.
- Check wiring and grounds to coils/injectors/sensors; repair any damaged connectors or poor grounds.
- After repairs or component swaps, clear codes and retest under the same conditions to confirm misfire is resolved. If intermittent, use extended logging/drive cycle.
- If no root cause is found, consider ECU software update or professional diagnosis with advanced tools.
Likely causes
- Worn/failed spark plugs or ignition coils
- Fuel delivery problem (low rail pressure or weak injectors)
- Vacuum/intake air leak causing lean condition
- Bad crankshaft or camshaft position sensor
- Low compression in one or more cylinders
Fault status
Status
Random/multiple cylinder misfire detected by PCM. The engine is misfiring intermittently or on multiple cylinders; further diagnosis required to identify whether ignition, fuel, air, mechanical, sensor, or wiring faults are responsible.
Repair difficulty: Medium
Diagnostic time: 1-4 hours (varies with cause and vehicle access)
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