Diagnostic procedure

WARNING: This page is about a different car, the 2005 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

DTC P0300 [AJ] DIAGNOSTIC PROCEDURE

STEP	INSPECTION		ACTION
1	VERIFY FREEZE FRAME DATA AND DIAGNOSTIC MONITORING TEST RESULTS HAVE BEEN RECORDED Have FREEZE FRAME DATA and DIAGNOSTIC MONITORING TEST RESULTS (Misfire related) been recorded?	Yes	Go to the next step.
1		No	Record FREEZE FRAME DATA and DIAGNOSTIC MONITORING TEST RESULTS on repair order, then go to the next step.
2	VERIFY RELATED REPAIR INFORMATION AVAILABILITY Check for related Service Bulletins and/or online repair information availability. Is any related repair information available?	Yes	Perform repair or diagnosis according to the available repair information. If the vehicle is not repaired, go to the next step.
2		No	Go to the next step.
3	VERIFY RELATED PENDING CODE OR STORED DTCs Turn ignition switch to OFF, then ON (Engine off). Verify related pending code or stored DTCs. Are other DTCs present?	Yes	Go to appropriate DTC troubleshooting. (See DTC TABLE .)
3		No	Go to the next step.
4	VERIFY CURRENT INPUT SIGNAL STATUS (KEY TO ON/IDLE) Access APP1, APP2, ECT, IAT, MAF, RPM, TP_REL and VSS PIDs using the WDS or equivalent. (See PCM INSPECTION .) Is there any signal that is far out of specification when ignition switch is ON and engine runs at idle?	Yes	Inspect suspected circuit or part or both according to inspection results. (See PCM INSPECTION .) Then go to Step 19.
4		No	Go to the next step.
5	VERIFY CURRENT INPUT SIGNAL STATUS UNDER TROUBLE CONDITION Inspect same PIDs as in Step 4 when simulating FREEZE FRAME DATA condition. Is there any signal which causes drastic changes?	Yes	Inspect suspected circuit or part or both according to inspection results. (See PCM INSPECTION .) Then go to Step 19.
5		No	Go to the next step.
6	INSPECT CMP SENSOR Inspect CMP sensor. (See CAMSHAFT POSITION (CMP) SENSOR INSPECTION .) Is there any malfunction?	Yes	Inspect installation condition and damages on timing belt and gears, repair faulty parts. If it is normal, replace CMP sensor. (See CAMSHAFT POSITION (CMP) SENSOR INSPECTION .) Then go to Step 19.
6		No	Go to the next step.
7	VERIFY CKP SENSOR INSTALLATION CONDITION Check for CKP sensor looseness. Is CKP sensor loose?	Yes	Retighten CKP sensor, then go to Step 19. (See CRANKSHAFT POSITION (CKP) SENSOR INSPECTION .)
7		No	Go to the next step.
8	INSPECT IGNITION SYSTEM OPERATION Perform Spark Test. (See SPARK TEST .) Is result of Spark Test normal?	Yes	Go to the next step.
8		No	Repair or replace malfunctioning part according to spark test result, then go to Step 19.
9	INSPECT MAF PID Start engine. Access MAF PID using the WDS or equivalent. Verify that MAF PID changes quickly according to race engine RPM. Is MAF PID response normal?	Yes	Go to the next step.
9		No	Replace MAF/IAT sensor, then go to Step 19. (See INTAKE AIR SYSTEM REMOVAL/INSTALLATION .)
10	INSPECT EXCESSIVE AIR SUCTION IN INTAKE AIR SYSTEM Inspect for air leakage at the following: Between MAF/IAT sensor and throttle body Between throttle body and dynamic chamber Is there any malfunction?	Yes	Repair or replace suspected part, then go to Step 19.
10		No	Go to the next step.
11	INSPECT FUEL LINE PRESSURE Turn the ignition switch off. Inspect fuel line pressure. (See FUEL LINE PRESSURE INSPECTION .) Is there any malfunction?	Yes	If fuel line pressure is too low, go to the next step. If fuel line pressure is excessively high, check as follows: Fuel pump maximum pressure Fuel return line for clogging If all above are normal, replace pressure regulator. (See PRESSURE REGULATOR INSPECTION .) Then go to Step 19.
11		No	Go to step 18.
12	INSPECT FUEL LINE FROM FUEL PUMP TO FUEL DELIVERY PIPE Visually inspect for fuel leakage in fuel line for any leakage. Is any fuel leakage found?	Yes	Replace suspected fuel line, then go to Step 19.
12		No	Inspect low-pressure side fuel filter for the following: Foreign material or stain inside fuel filter Perform the following actions as result: If foreign material or stain is found inside fuel filter (low-pressure side), clean of fuel tank and filter. If normal, replace fuel pump unit. (See FUEL PUMP UNIT REMOVAL/INSTALLATION .) Then, go to Step 22.
13	INSPECT ENGINE COMPRESSION Inspect engine compression. (See COMPRESSION INSPECTION .) Is it normal?	Yes	Go to the next step.
13		No	Implement engine overhaul for repairs, then go to Step 19.
14	INSPECT VARIABLE VALVE TIMING CONTROL SYSTEM OPERATION Inspect variable valve timing control system operation. (See VARIABLE VALVE TIMING CONTROL SYSTEM OPERATION INSPECTION .) Does variable timing control system work properly?	Yes	Go to the next step.
14		No	Repair or replace malfunctioning part according to variable valve timing control system inspection results, then go to Step 19.
15	INSPECT OPERATION OF PURGE CONTROL SOLENOID VALVE Inspect purge control system operation. (See PURGE CONTROL SYSTEM INSPECTION .) Does purge control system work properly?	Yes	Go to the next step.
15		No	Repair or replace malfunctioning part according to purge control system inspection results, then go to Step 19.
16	INSPECT PCV VALVE OPERATION Turn the ignition switch off. Remove PCV valve and check valve operation. (See POSITIVE CRANKCASE VENTILATION (PCV) VALVE INSPECTION [AJ] .) Is PCV valve operation normal?	Yes	Replace PCV valve, then go to Step 19.
16		No	Go to the next step.
17	INSPECT OPERATION OF EGR VALVE Remove EGR valve. See EGR VALVE REMOVAL/INSTALLATION [AJ] .) Visually check whether it is stuck to open. Is EGR valve stuck to open?	Yes	Go to the next step.
17		No	Repair or replace EGR valve, then go to Step 19. (See EGR VALVE REMOVAL/INSTALLATION [AJ] .)
18	INSPECT SEALING OF ENGINE COOLANT PASSAGE WARNING: Removing radiator cap when radiator is hot is dangerous. Scalding coolant and steam may shoot out and cause serious injury. When removing radiator cap, wrap thick cloth around and turn it slowly. Remove radiator cap. Implement procedure to bleed air from engine coolant, then run engine at idle. Are there any small bubbles which make engine coolant white at filling opening? NOTE: Large bubbles are normal since they are remaining air coming out from engine coolant passage.	Yes	Air gets in from poor sealing of head gasket or other areas between combustion chamber and engine coolant passage. Repair or replace faulty parts, then go to the next step.
18		No	Go to the next step.
19	VERIFY TROUBLESHOOTING OF DTC P0300 COMPLETED Make sure to reconnect all disconnected connectors. Turn the ignition switch to the ON position. Clear the DTC from the memory using the WDS or equivalent. Start engine and perform DRIVE MODE 1. (See OBD-II DRIVE MODE .) Is PENDING CODE same as DTC present?	Yes	Replace PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION .)
19		No	Go to the next step.
20	VERIFY AFTER REPAIR PROCEDURE Perform "AFTER REPAIR PROCEDURE". (See AFTER REPAIR PROCEDURE .) Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE .)
20		No	Troubleshooting completed.