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On late model OBD2 cars how does the ECU register a misfire? Also how is it the ECU recognizes the difference between knock and an ignition misfire?

On late model OBD2 cars how does the ECU register a misfire? Also how is it the ECU recognizes the difference between knock and an ignition misfire?

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OBD reports to the scan tool in different "modes". On most cars, misfires are reported in mode 6. The DTC that is stored in memory when the MIL comes on also reports in cylinder-specific codes. P0301 would indicate a misfire on #1 cylinder, P0302 #2 cylinder, etc. The late model OBD2 systems also have a "Knock Sensor" which sets a P0325 code when it returns a signal outside of parameter. It is mounted somewhere into the engine block (sometimes requiring disassembly of components to get to it). The Knock Sensor electronically signals the computer to retard ignition timing, enrich fuel or other strategies aimed at eliminating pre-detonation knock instantly.

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Jim O’Neill
Chino Autotech Inc.
Auto Club SoCal AAR

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