Random combustion misfire detected


Detection Condition

Diagnostic Support Notes

Possible Causes


Diagnostic Procedure

1.  Has FREEZE FRAME DATA been recorded?

2.  Check for related TSBs and/or online repair information.  Is any repair information available?

3.  Turn ignition key to OFF then ON (engine off).  Any other pending or current DTCs present?

4.  With an OBD interface, access engine coolant temperature (ECT), intake air temperature (IAT), mass airflow (MAF), engine revolutions per minute (RPM), throttle position (TP), and vehicle speed (VS) PIDs.  Is there any signal that is far out of specification when ignition switch is ON and engine is idling?

5.  Inspect same PIDs as in Step 4 while simulating FREEZE FRAME DATA condition. Is there any signal which causes drastic changes?

6.  Turn ignition key to OFF.  Inspect the camshaft position sensor.  Is camshaft position sensor okay?

7.  Check crankshaft position sensor for looseness.  Is crankshaft position sensor loose?

8.  Verify proper ignition system operation using a timing light.  Do all cylinders exhibit proper ignition system operation?

9.  Turn ignition key to OFF and inspect the high tension leads (KL) or coil-on-plug units (KJ) for damage, wear, improper installation, corrosion on terminal, open leads, etc.  Are all items okay?

10.  Disconnect ignition coil connector (KJ).  Turn ignition key to ON (engine OFF) and measure the voltage between ignition coil terminal D (harness-side) and body ground.  Is voltage reading B+?

11.  Check ignition coil resistance.  Is coil resistance okay?

12.  Start engine and access MAF PID using an OBD interface.  Verify that MAF PID changes quickly as the engine is raced in Park or Neutral.  Is MAF PID response okay?

13.  Inspect for air leakage between MAF sensor and throttle body or between throttle body and dynamic chamber (KJ).  Are there any leaks present?

14.  Inspect fuel line pressure.  Is fuel line pressure okay?

15.  Disconnect vacuum hose from pressure regulator and start engine.  Is vacuum felt at the open end of the vacuum hose?

16.  Inspect fuel pump maximum pressure.  Is fuel pump maximum pressure within 630-800 kPa (92-116 psi)?

17.  Visually inspect for fuel leakage from fuel line.  Is any fuel leakage found?

18.  Inspect engine compression.  Is engine compression okay?

19.  Perform purge solenoid valve inspection.  Is the purge solenoid valve okay?

20.  Turn ignition key to OFF, remove the PCV valve and check for proper operation.  Is PCV valve operation okay?

21.  Remove EGR valve and perform a visual check to ensure the valve is not stuck open.  Is the EGR valve stuck open?

22.  With the engine sufficiently cool, remove the radiator cap, implement coolant system bleed procedure, then idle the engine.  Are there any small bubbles, which make the engine coolant appear white at the filler opening?

23.  Make sure to reconnect all disconnected connections.  Turn ignition key to ON (engine OFF).  Clear DTC from memory using OBD interface.  Start engine and perform DRIVE MODE 1.  Is PENDING CODE for P0300 present?

24. Perform "After Repair Procedure" and scan for stored codes.  Is there a DTC present?


[P0300 - From 2002 Mazda Millenia factory service manual]