One very important point to keep in mind about OBD II misfire detection is that the system does not tell you why the engine is misfiring. The OBD II misfire monitor is normally active when a warm engine is running under normal loads, but it may not track misfires if other fault codes are present or codes are pending for the fuel system, evaporative emission control (EVAP) system or EGR system. This helps avoid the setting of false misfire codes, and also allows the system to spot pattern misfires that may cause an increase in emissions or damage to the catalytic converter. By averaging the pass/fail ratings of all 16 blocks, the OBD II system gets the big picture of what’s actually going on. But if the number of misfires in that block is too high, the block receives a “fail” rating. The tally is stored in 16 memory blocks, so every 3,200 rpm the misfire count starts over again.Īs long as the number of misfires in any given memory block remains below a certain value, there is no problem and OBD II gives that block a “pass” rating. Any misfires that occur are recorded every 200 revolutions of the crankshaft. On most applications, OBD II uses a “block learn” strategy to track misfires. And if a misfire is random and jumps from cylinder to cylinder, you may find a P0300 random misfire code. If more than one cylinder is misfiring, there will be additional codes for those cylinders as well.
MONITOR MISFIRES WITH SCAN XL PROFESSIONAL CODE
A code P0302, for example, would tell you cylinder number two is misfiring.