A Man For All Autos

By John Hummel

I once had a customer come into my shop with one of those intermittent problems. We went through all of the pinpoint tests and compared data with known good units. I explained what should be done and he left. Several days later this customer called to tell me another mechanic had plugged in a machine that told him to replace a certain part. He thought that I should consider buying one of those machines because it would tell me what to replace without having to know anything. I thanked him for the information and promptly called the individual he told me supplied the crystal ball. When I asked if this store had such a machine, and that I would be willing to pay dearly for it, they laughed. They commented they would like one also.

Many times my customers have asked me, why is it so hard to diagnose today's car? "Can't you just plug in a computer and it will tell you what to replace?" I only wish it were that easy! Computers have made many aspects of diagnostics easier.

However, they have made other aspects more difficult. When the tech plugs in his scanner he is given access to the memory of the computer. If at anytime in the reasonable past the car's computer has detected a problem in an emission circuit, then it will set a code. The codes are like an index to an encyclopedia. You wouldn't want to look up the subject of physics under the heading of zoology. When the codes are read using the scanner, it gives the tech a starting place. He then can go to the "troubleshooting tree" and by following the prescribed steps, he can find out what is causing the problem.

On the more modern vehicles, codes can be set that are triggered by a malfunction in another circuit. For instance, a plugged fuel filter can cause and oxygen sensor code to set, or a system lean code. These problems require that the tech have a thorough understanding of how the system interacts with other systems to accurately diagnose the problem.

I hope this will clear up a little of the misunderstanding about how trouble codes work.