What is the deal with trouble codes anyway?

 One of the most common falsehoods you'll hear concerning car repair is that trouble codes will tell you what is bad on your car. Just read the code and know what the problem is. There is no truth in that statement and, for some reason many people believe it anyway. I imagine it gets tossed out there most often by misleading ads from makers of code readers. I can tell you this, if a code reader would diagnose a car problem I wouldn't spend any time learning how systems work, how to test operation or definitely wouldn't spend so much money on testing equipment. 

If you were driving a car and noticed a problem that you felt should be checked out, you would bring the car in and describe what you are noticing. Coming in and saying something is looking like a problem on my car, with no more information, then I'd have a really hard time determining what you are seeing that you are worried about. A warning light coming on in your car is the equivalent of that. The light is just saying hey I noticed something. If it stopped there I would have an extremely hard time determining the problem. If you said I noticed a problem with the engine sometimes being hard to start, idling rough and black smoke, then I would know what sort of things to start checking to determine the problem. Trouble codes are the way the car tells you more about what it is seeing that looks like a problem. It is giving you a place to start testing to determine the problem. 

If I need more information from you about the problem, I ask for it. If I need more from the car computer, I use a scan tool to look a bit further. So, I ask for it. 

Let's walk through one. 2012 Ford Edge Limited 3.5 Because the Malfunction Indicator Light is on and because that just means there is a problem, I need codes. I don't need codes to tell me what is wrong, because they don't. But I do need codes to know what sort of concerns the computer has noticed. 

The car says, I have noticed the fuel mixture is rich at times on both engine banks. My job then becomes determining why the computer is seeing rich fuel mixture. I can ask questions with the scan tool and look for more data such as what conditions were present last time the problem was seen (freeze frame), if the problem is present at this time (scan data) and look for common causes (TSBs). So if I'm looking at a scan tool for a bit, it isn't because I am being told what the car needs, it is because I am using the stored data to get more information so that I can determine what is causing the problem. In this particular case, the Evaporative Emission Purge Valve was stuck open and drawing fuel vapors into the engine when it should not have been. The O2 sensors noticed the resulting rich fuel mix and set the rich codes. 

Testing the valve found the problem. Replacing the valve repaired it. 

Kenneth Hayes

G&G Auto Repair

Kenny@ggauto.repair