Circuit description: Notes

When a misfire occurs in the engine, hydrocarbon (HC) is released into the exhaust gas. If the HC concentration increases, the exhaust emission level deteriorates. At the same time, the increase in HC concentration causes an increase in catalyst temperature, and may degrade the purification performance of the catalyst. In order to prevent an increase in exhaust emissions and avoid the thermal deterioration of catalysts, the engine control computer monitors the misfire rate (number of misfires divided by number of ignitions). To monitor misfires, the engine control computer uses the camshaft position sensor and crankshaft position sensor. The crankshaft position sensor is used to measure the crankshaft revolution speed. When the variation of crankshaft revolution speeds exceeds the specified value, the engine control computer determines the vehicle is misfiring, and counts the misfire count (total number of misfires).

The camshaft position sensor is used to identify which cylinders are misfiring.

Courtesy of SUBARU OF AMERICA, INC.

DTC No.	DTC detection conditions	Suspected parts
P0300	When multiple cylinders misfire simultaneously, and one of the following applies (2 trip detection logic): Occurrence of a misfire that may damage the three-way catalytic converter (MIL flashes) Occurrence of a misfire that causes the exhaust amount to deteriorate (MIL illuminates)	Open/short circuit in the engine wiring harnesses Connectors connection Vacuum hose connection Ignition system Fuel injector ASSY (port injection) Fuel injector ASSY (cylinder injection) Fuel pressure Intake air flow meter SUB-ASSY E.F.I. water temperature sensor Compression pressure Valve timing PCV valve and hose PCV hose connection Intake system ECM
P0301	When No. 1 cylinder misfires simultaneously, and one of the following applies (2 trip detection logic): Occurrence of a misfire that may damage the three-way catalytic converter (MIL flashes) Occurrence of a misfire that causes the exhaust amount to deteriorate (MIL illuminates)
P0302	When No. 2 cylinder misfires simultaneously, and one of the following applies (2 trip detection logic): Occurrence of a misfire that may damage the three-way catalytic converter (MIL flashes) Occurrence of a misfire that causes the exhaust amount to deteriorate (MIL illuminates)
P0303	When No. 3 cylinder misfires simultaneously, and one of the following applies (2 trip detection logic): Occurrence of a misfire that may damage the three-way catalytic converter (MIL flashes) Occurrence of a misfire that causes the exhaust amount to deteriorate (MIL illuminates)
P0304	When No. 4 cylinder misfires simultaneously, and one of the following applies (2 trip detection logic): Occurrence of a misfire that may damage the three-way catalytic converter (MIL flashes) Occurrence of a misfire that causes the exhaust amount to deteriorate (MIL illuminates)