Figurin’ out what’s wrong

One of the most misunderstood functions by consumers of automotive repair is the “diagnostic test”. Many people have the misconception that the “computer” tells the technician what is wrong with the vehicle. A smaller percentage of people believe that the computer even forecasts what is going to go wrong in the upcoming several weeks. How I wish either of those beliefs were actually true. It would make our business so much simpler.

The Check Engine Light or Service Engine Soon Light is regulated by Federal Law. It cannot come on at predetermined mileages, nor can it come on only because a recommended maintenance service was not performed. This light comes on when a process condition exceeds +/- 50% of its normal range. Many of the conditions monitored will not affect the way the car drives, and only affect the amount of pollution produced. A very few of the monitored conditions can indicate imminent engine or transmission damage. And yes, the light may come on simply because you forgot to tighten the gas cap. (When the cap is left loose an emission test runs and fails because the cap is loose.)

When the vehicle computer turns the Check Engine Light on the computer also stores a Trouble Code as well as some engine operating information at the moment that the code set.

The Diagnostic Test begins by reading the Trouble Code that was set. Often the light came on because a sensor failed. The actual operation of the engine did not change, only the sensor failed. Unfortunately the “computer” cannot determine whether the sensor failed or the condition is really present.

If you do not believe this, ask yourself how many times you or a friend have had a repair only to have the light come back on with the same code. If the computer were so good at determining the problem it would only be sight-impaired technicians who could not fix it on the first repair.

What should happen during diagnostic testing is that the technician should check for trouble codes first, then look at a half dozen other process conditions with the engine running. The technician should look at the output of the sensor indicated by the code and see if those readings are consistent with the other readings or not. Then the technician should make specific changes in conditions, such as by adding propane gas or creating a vacuum leak. Even when doing this the problem is not always obvious.

The best automotive technicians are always going to new training classes. The classes primarily deal with new technologies in how the car’s computer uses the sensors to control the engine, and how to accurately diagnose the problem. We also have to continually buy new expensive diagnostic computers to get better information from the car’s computer. Each of these diagnostic computers cost $4,000 to $8,000. We have bought seven in the past four years. Each computer has an annual software update cost. We spend $4,500 each year on software updates.

The trouble codes were defined in 1996, and have not changed since. If it were as simple as reading the trouble code we wouldn’t go to the trouble and expense of all of the training, equipment, and software. Unfortunately, even with all of the training and equipment and all of the care and concern, technicians are not always able to completely diagnose the problem on the first try. It is not necessarily an indication of incompetence or error, but rather that they are working with a very complicated engine control process.

Brian Klaubert has written for the Neighborhood News for five years, and owns Christian Brothers Automotive – Hamilton Mill on Braselton Hwy in Dacula. They have been voted Best of Gwinnett in both the Gwinnett Daily Post and Gwinnett Magazine.