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?

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?

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.

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

You May Also Like

MAF and MAP Sensors

These small-but-mighty components play an outsized role in keeping fuel-injected engines running smoothly.

MAF and MAP Sensors

While it might not sound like it to the untrained ear, the orchestration of components to achieve the ideal combustion cycle is nothing short of a symphony.

For fuel-injected engines, two important instruments in this precise arrangement are the mass airflow (MAF) sensor and the manifold absolute-pressure (MAP) sensor.

Electronic Parking Brakes

Safety, convenience and holding power – what’s not to love?

Electronic Parking Brake
Driveshaft Dynamics

Don’t let the terminology trip you up.

Driveshaft
Serpentine Belts Have a Strong Supporting Cast

Tensioners, balancers and pulleys are working behind the scenes to maintain harmony under the hood.

Tensioners and Pulleys
Artificial Intelligence in the Automotive Aftermarket

The applications for AI are endless, but hurdles still remain.

AI Aftermarket

Other Posts

Interpreting Dashboard Warning Lights

Navigating the neon jungle isn’t as easy as it used to be.

Dashboard Diagnostics
Stopping Power: Brake Master Cylinders

The brake master cylinder is the hydraulic hero behind every safe stop.

Brake Master Cylinder
Winter Vehicle Prep

Keep your customers safe with these preventative-maintenance tips.

Deer-Collision Season: It’s Bucks vs. Bumpers

In the heart of deer-collision season, drivers need to be on high alert.