DTC P0300: Random/Multiple Cylinder Misfire Detected: Notes

DTC detecting condition:

Detected when two consecutive driving cycles with fault occur.
Immediately at fault recognition (A misfire which could damage catalyst occurs.)

Trouble symptom:

Engine stalls.
Improper idling
Rough driving
CAUTION:
- After servicing or replacing faulty parts, perform Clear Memory Mode Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>CLEAR MEMORY MODE>OPERATION , and inspection mode Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>INSPECTION MODE>PROCEDURE ..
- Use the check board when measuring the ECM terminal voltage and resistance.

Wiring diagram:

Engine Electrical System Refer to WIRING SYSTEM>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

CHECK HARNESS BETWEEN ECM AND FUEL INJECTOR CONNECTOR.
1. Turn the ignition switch to OFF.
2. Disconnect the connector from all fuel injectors.
3. Measure the resistance between all fuel injector connectors and engine ground.
  Connector & terminal
  
  #1 (E5) No. 1 - Engine ground:
  
  #1 (E5) No. 2 - Engine ground:
  
  #2 (E16) No. 1 - Engine ground:
  
  #2 (E16) No. 2 - Engine ground:
  
  #3 (E6) No. 1 - Engine ground:
  
  #3 (E6) No. 2 - Engine ground:
  
  #4 (E17) No. 1 - Engine ground:
  
  #4 (E17) No. 2 - Engine ground:
Is the resistance 1 MΩ or more?

Yes: Go to 2.

No: Repair the short circuit to ground in harness between ECM connector and fuel injector connector.
CHECK HARNESS BETWEEN ECM AND FUEL INJECTOR CONNECTOR.
Measure the resistance of harness between ECM connector and fuel injector connector on all cylinders.

Connector & terminal

#1 (E158) No. 12 - (E5) No. 1:

#1 (E158) No. 11 - (E5) No. 2:

#2 (E158) No. 24 - (E16) No. 1:

#2 (E158) No. 23 - (E16) No. 2:

#3 (E158) No. 36 - (E6) No. 1:

#3 (E158) No. 35 - (E6) No. 2:

#4 (E158) No. 48 - (E17) No. 1:

#4 (E158) No. 47 - (E17) No. 2:

Is the resistance less than 1 Ω?

Yes: Go to 3.

No: Repair the open circuit of the harness between the ECM connector and fuel injector connector.
CHECK FUEL INJECTOR.
Check all fuel injectors. Refer to FUEL INJECTION (FUEL SYSTEMS) (EXCEPT FOR STI)>FUEL INJECTOR>INSPECTION .

Is the check result OK?

Yes: Go to 4.

No: Replace the faulty fuel injector. Refer to FUEL INJECTION (FUEL SYSTEMS) (EXCEPT FOR STI)>FUEL INJECTOR .
CHECK INSTALLATION CONDITION OF CAMSHAFT POSITION SENSOR/CRANKSHAFT POSITION SENSOR.
Check installation condition of the camshaft position sensor and crankshaft position sensor.

Is the camshaft position sensor or crankshaft position sensor loosely installed?

Yes: Tighten the camshaft position sensor or crankshaft position sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (EXCEPT FOR STI)>CAMSHAFT POSITION SENSOR>INSTALLATION . Refer to FUEL INJECTION (FUEL SYSTEMS) (EXCEPT FOR STI)>CRANKSHAFT POSITION SENSOR>INSTALLATION .

No: Go to 5.
CHECK CRANKSHAFT POSITION SENSOR PLATE.
Check the crankshaft position sensor plate.

Is the crankshaft position sensor plate rusted or does it have broken teeth?

Yes: Replace the crankshaft position sensor plate. Refer to MECHANICAL (EXCEPT FOR STI)>CYLINDER BLOCK .

No: Go to 6.
CHECK INSTALLATION CONDITION OF TIMING CHAIN.
Turn the crankshaft using ST, and align the alignment mark on crank sprocket with alignment mark on cylinder block.

ST 18252AA000 CRANKSHAFT SOCKET

Is the timing chain dislocated from its proper position?

Yes: Correct the installation condition of timing chain. Refer to MECHANICAL (EXCEPT FOR STI)>TIMING CHAIN ASSEMBLY .

No: Go to 7.
CHECK FUEL LEVEL.
Inspect the fuel level.

Is the fuel meter indication higher than the "Lower" level?

Yes: Go to 8.

No: Refill the fuel so that the fuel meter indication is higher than the "Lower" level, and proceed to the next step. Go to 11.
CHECK PCV VALVE.
Check the PCV valve. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (EXCEPT FOR STI)>PCV VALVE>INSPECTION .

Is the check result OK?

Yes: Repair or replace the PCV valve. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (EXCEPT FOR STI)>PCV VALVE .

No: Go to 9.
CHECK PCV HOSE ASSEMBLY.
Check the PCV hose assembly.

Is the PCV hose assembly deformed, cracked or damaged?

Yes: Repair or replace the PCV hose assembly. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (EXCEPT FOR STI)>PCV HOSE ASSEMBLY .

No: Go to 10.
CHECK PCV HOSE ASSEMBLY.
Check the clamp of PCV hose assembly.

Is the clamp of PCV hose assembly damaged or loose?

Yes: Repair or replace the clamp of PCV hose assembly. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (EXCEPT FOR STI)>PCV HOSE ASSEMBLY .

No: Go to 11.
CHECK STATUS OF MALFUNCTION INDICATOR LIGHT.
1. Clear the memory using the Subaru Select Monitor or general scan tool. Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>CLEAR MEMORY MODE .
2. Start the engine, and drive the vehicle for 10 minutes or more at 60 km/h (37.3 MPH).
Does the malfunction indicator light illuminate or blink?

Yes: Go to 12.

No: Repair the poor contact of connector.
NOTE:
In this case, repair the following item:
- Poor contact of ignition coil connector
- Poor contact of fuel injector connector on faulty cylinders
- Poor contact of ECM connector
- Poor contact of coupling connector
CHECK AIR INTAKE SYSTEM.
Check installation condition of the air intake system.

Is there any fault in air intake system?

Yes: Repair the air intake system.
NOTE:
Check the following items.
- Are there air leaks or air suction caused by loose or dislocated nuts and bolts?
- Are there cracks or any disconnection of hoses?
No: Go to 13.
CHECK ALL CYLINDERS.
Check the following items for the cylinders.
- Spark plug
- Ignition coil
- Fuel injector
- Compression
- Skipping timing chain teeth
Is there a fault in any cylinder?

Yes: Repair or replace the faulty part of the faulty cylinder.
NOTE:
Check the following items.
- Spark plug
- Ignition coil
- Fuel injector
- Compression
- Skipping timing chain teeth
No: Go to DTC P0171. Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)>DTC P0171 SYSTEM TOO LEAN BANK 1 .