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Focus on CAN Diagnostics


Several months ago, we featured our first article that addressed CAN network diagnosis. We received a large amount of feedback on that one, so we thought we would provide additional resources related to diagnosing and troubleshooting vehicle communication issues. As always, we have turned to some of the top trainers and writers in the business to bring you a roundup that covers the top issues related to this service area.

CAN Network Overview
In this network primer from Import Car, Gary Goms provides a history of vehicle networks, an overview of today’s CAN network designs and a detailed look at the DLC and its function. A great place to start our network journey, this article is quick and provides a great introduction to the main concepts of vehicle networks and their component parts. He provides a quick look at U Codes, along with a word of warning: “I’ve found that U-code diagnostics should be approached with caution because a simple problem like a worn ignition switch can set “U” codes by intermittently reducing voltage to the modules.”

Implement a Plan of Attack

It seems to always come back to a few fundamental principles that we have observed as nearly mantra-like among the acknowledged exerts in the field of electrical and network system diagnosis. One of the most common of these principles is to have a specific plan of attack and consistently follow that plan each time you perform a diagnostic search and destroy mission. In this article on, Scott Shotton adds the qualifier “logical” to this axiom: follow a “logical” approach. Makes sense. Scott outlines a four step plan to get pointed in the right diagnostic direction. He then provides two case studies, showing how the four step plan was used to resolve a network issue. As he says, “following the four step plan outlined here will direct a technician through an efficient and effective diagnostic process.”

Tackling U Codes and
Non-Communication Errors

In a nice complement to the above article, Eric Ziegler, on, extends the discussion with additional strategies for solving communications issues. He also stressed the need for a solid plan of attack (POA) when dealing with suspected vehicle network issues, particularly U Codes and other non-communication issues. He provides an illustration of a single non-communicating module as well as a multiple module issue. In addition, he touches on how to incorporate your scan tool and lab scope into your plan of attack.

Mastering a Critical Instrument – The Scope
When diagnosing communications issues, the trail often leads the technician to suspect a problem signal from one part of the system to another. This usually results in the need to scope the signal, as shown in the above articles. In this article, from, Dave Hobbs provides a detailed overview of scope usage, providing several examples of diagnostic investigation where the use of a scope proves instrumental. While most of the examples are electrical system diagnostic dilemmas, he closes with these words on communication issues: “If you’re working on a multiplexing or bus communications problem, you can use a DSO to verify the presence or absence of a bus communications signal. Whatever the case, a modern DSO is the indispensable tool for the modern diagnostic technician.”

An Oldie But a Goodie
This video is older (about 10 years old), but if you are looking for thorough, detailed introduction to vehicle communication systems, here’s a great place to start. In this video of a training seminar, Dave Hobbs reviews the history of vehicle networks, provides a detailed explanation of network function, breaks down the various components of vehicle networks and their interactions, and then gets into specifics. Again, some of the aspects of this lecture are dated (scan tools referenced, etc.), but if your goal is to better understand how vehicle networks operate, this is the one for you.

We hope that this review is helpful to you. If we use trade publication activity as a measure of how widespread the call is for more information on a given topic, it would appear that the demand for greater knowledge of network diagnosis and repair strong. This makes sense, as network diagnostics is an increasingly critical skill needed by today’s technician in solving a variety of repair issues. A skill we suspect will only grow in importance as system complexity increases.

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