General Information

P0303 Cylinder 3-Misfire detected

General Information

General Description

Misfires can be caused by lack of combustion in a cylinder due to absence of spark, poor fuel metering, poor compression, or many other causes. Even a small number of misfires may result in excessive exhaust emissions due to the unburned mixture. Increased misfire rates cause damage to the catalytic converter. The PCM monitors the crankshaft speed variation to determine if any misfiring generated. The PCM identifies the specific cylinder in which the misfire has occurred and counts individual misfire events by monitoring changes in the crankshaft rotation for each cylinder. A random misfire indicates two or more cylinders are misfiring.

DTC Description

The PCM measures reference event times and calculates the positive and negative acceleration of the crank wheel to determine whether a misfire has occurred.
When the rate of misfire exceeds a threshold where the catalyst reaches a temperature where permanent damage can occur, to the point that tail pipe emissions reach 1.5 times the tailpipe standard or where a cylinder misfire cause a loss of torque produced from that cylinder. PCM sets this DTC. In case that misfire affects Catalyst damage, MIL (Malfunction Indication Lamp) will be illuminating and blinking at 1 HZ frequency. However, In case of Individual and Emission damaging misfire, MIL will be turned on when the malfunction is detected.

DTC Detecting Condition





Signal Waveform & Data








Fig.1) Normal data of misfire counter at idle.
Fig.2) Abnormal data of misfire counter when cylinder 4 injector open.