When the engine misfires, high concentrations of hydrocarbons (HC) enter the exhaust gas. Extremely high HC concentration levels can cause increase in exhaust emission levels. High concentrations of HC can also cause increases in the Three-Way Catalytic Converter (TWC) temperature, which may cause damage to the TWC. To prevent this increase in emissions and to limit the possibility of thermal damage, the ECM monitors the misfire rate. When the temperature of the TWC reaches the point of thermal degradation, the ECM blinks the MIL. To monitor misfires, the ECM uses both the Camshaft Position (CMP) sensor and the Crankshaft Position (CKP) sensor. The CMP sensor is used to identify any misfiring cylinders and the CKP sensor is used to measure variations in the crankshaft rotation speed. Misfires are counted when the crankshaft rotation speed variations exceed predetermined thresholds. If the misfire exceeds the threshold levels, and could cause emission deterioration, the ECM illuminates the MIL and sets a DTC.


DTC Detection Conditions

Trouble Areas


P0301 P0302 P0303 P0304 P0305 P0306

Simultaneous misfiring of several cylinders detected (2 trip detection logic)

Misfiring of specific cylinder detected (2 trip detection logic)

Open or short in engine wire harness

Connector connection

Vacuum hose connections

Ignition system


Fuel pressure

Mass Air Flow (MAF) meter

Engine Coolant Temperature (ECT) sensor

Compression pressure

Valve clearance

Valve timing

PCV valve and hose

PCV hose connections

Air induction system


When DTCs for misfiring cylinders are randomly set, but DTC P0300 is not set, it indicates that misfires have been detected in different cylinders at different times. DTC P0300 is only set when several misfiring cylinders are detected at the same time. Reference: Inspection using an oscilloscope.

With the engine idling, check the waveform between terminals #10 to #60 and E1 of the ECM connectors.




#10 to #60 - E01

Equipment Settings

20 V/Division, 100 or 1 ms/Division




The correct waveform is as shown.

Injector Signal Waveform








100 msec./DIV. (Idling)

Injection Duration 1 msec./DIV. (Idling)



The ECM illuminates the MIL and sets a DTC when either one of the following conditions, which could cause emission deterioration, is detected (2 trip detection logic).

• Within the first 1,000 crankshaft revolutions of the engine starting, an excessive misfiring rate (approximately 20 to 50 misfires per 1,000 crankshaft revolutions) occurs once.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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