Q: I’ve been working at a repair shop for about six months. Recently, my boss let me start doing some diagnostics, using a diagnostic flow chart as a guide. Unfortunately, my results have been mixed. Are there any books dedicated to diagnosing fault codes? Do you have any suggestions for improving my diagnostic skills? One vehicle that has been in the shop for quite a while is a C4 with a “Throttle Position” code that no one can seem to fix. I would like to repair this car to gain some confidence, but I don’t know where to start.
A: I’m afraid there are very few praiseworthy publications for diagnosing fault codes. Most suggested diagnostic procedures fall into one of three categories, or a combination of the three. The first method involves a step-by-step diagnostic flow chart. Once you have acquired the fault code, this procedure takes you through a series of questions. Each answer directs you to another question, until you reach the most likely cause of the problem. Unfortunately, if you answer even one question incorrectly, you’ll end up with the wrong diagnosis, or multiple recommendations.
The second method involves simply replacing the part indicated by the fault code. For example, if the fault in question indicates a throttle-position-sensor problem, just replace the throttle-position sensor.
The third method involves checking the Internet to see what other people have done to repair a similar problem. This is actually a good idea if you use the information you find as a tool, and not your sole troubleshooting source. (Yes, it’s true: Not all of the information on the Internet is accurate.)
The problem with the first two methods is that they don’t help you learn how a system works. This is evident in other areas of the auto-repair field. For example, when it comes to failures in components such as alternators and starters, many of today’s automotive technicians do not know what, precisely, has failed. All they know is that something is wrong, so they replace the part. Older automotive technicians generally know how subcomponents such as alternators and starters work, because when they were younger, they actually rebuilt them. This experience often makes them better at diagnosing problems with such items. Knowledge, after all, is power.
There is a simple solution. Instead of learning how to follow a flow chart, try to learn the theory behind how a system works. Even if you still need to refer to a flow chart for help, knowing how the system you’re diagnosing functions will help you understand why you’re performing the specific tasks specified in the chart. This will ultimately make you a better technician.
The obvious benefit of this type of learning is that you only need to learn a few basic concepts in order to become proficient.
Research common failures: The first step in this technique is to check for any manufacturer Technical Service Bulletins (TSBs). This is also a good time to refer to the Internet to see what other people have done to repair a similar problem. There are also several pay sites that are excellent reference points, such as Alldata and Identifix.
Research how a system works: You’ll need to know how the system you are working with is supplied power, and how much. Also, what result is that system is trying to achieve?
Find your jumping off point: This is the point where you begin your diagnostics. The best jumping-off point is the most common failure item, as indicated by your research. By testing this device, you can determine whether it has failed, and which path you’ll need to take next.
Perform your diagnostics: Let’s begin by taking a look at a few of the sensors the engine controller uses to determine how it should perform in a given situation. A sensor is simply an input device that monitors driver inputs as well as engine temperatures, fuel mixture, and other parameters.