The Rules of Engine Diagnostics

July 11, 2022

It’s a rare tire dealership that does not occasionally see a car with an illuminated check engine light. In fact, 65% of respondents to MTD’s latest Tire Dealer Automotive Service Study report that they offer engine diagnostic services.

In this article, representatives from diagnostic tool providers discuss key things technicians should remember when trying to figure out why that check engine light is on.

Autel: “The most important thing for a technician to do in this instance is not to assume that the check engine malfunction light is caused by a fault in the engine system,” says Maurice Miller, Autel’s director of technical support.

“There’s an old adage that vehicle systems tell on each other, meaning a code may be tripped for a system while the actual fault or failure is in another.”

Because of this, “it’s always best practice to perform a scan of all the vehicle’s available systems,” he notes.

Bosch: “Vehicles are becoming increasingly technical, relying on sensors, electronics, and communication wires to operate appropriately,” says Duane Watson, technical trainer at Bosch.

“This poses new complexities to technicians and changes the way technicians perform routine services.

“Before performing any diagnostic check, technicians must ensure that their tools are up-to-date with the latest software.” Bosch regularly releases new updates for its diagnostics tools, he adds.

“If the software is outdated, it could impact the accuracy of readings and, in turn, make it increasingly difficult to diagnose the actual problem.

“Another important thing to remember is that shops are challenged with new, complex systems that could potentially create problems that certain technicians have never encountered.

“For example, a vehicle owner might bring their truck into the shop with a complaint or issue and when a technician goes to check and duplicate the problem, it no longer exists. “It doesn’t mean that there isn’t an issue with the vehicle. It means that figuring out the problem will be more complex.

“Technicians can face these challenges head-on with proper training and up-to-date diagnostic tools,” according to Watson.

Continental: Technicians should always follow the diagnostic process and use the same strategy-based diagnostic process for every check engine light or drivability concern, says John Sears, a senior member of the Continental technical support team.

“At Continental, we call It the three Cs - concern, cause and correction.” Sears recommends the following steps when attempting to determine why a check engine light is illuminated:

1.Talk with the customer and gather information. “Is the problem intermittent or does it occur all of the time? Does it happen when the engine is cold or hot? Did someone else work on the vehicle recently? Don’t be surprised if the customer is not completely honest about this one, especially if they worked on the vehicle themselves.”

2. Verify the concern. “Drive the vehicle,” says Sears. “You can’t fix the problem if it isn’t there while you’re working on it. If the complaint is an intermittent problem, go for a ride with the customer. Their driving style may be different than yours and that may shed some light on the issue.”

3. Perform a quick visual inspection. “Many problems can be found quickly by opening the hood and taking a look around with a flashlight.”

4. Retrieve any DTCs with a scan tool. “ If multiple DTCs are present, you should concentrate on the first one on the list.”

5. Examine scan tool data carefully. “Concentrate only on the critical data related to the DTC or the drivability concern,” says Sears. “This will help narrow the focus to the root cause of the concern. Remember that some driveability concerns will have no DTCs present.”

6. Repair the cause of the concern and verify the repair. “Clear fault codes and drive the vehicle for the correct drive cycles needed — remembering that some fault codes do not set on the first road test. You don’t want to give the vehicle back to the customer only to have the (light) come back on the next day while they are driving it.”

About the Author

Madison Gehring | Associate Editor

Madison Gehring is Modern Tire Dealer's associate editor. A graduate of Ohio State University, Gehring holds a bachelors degree in journalism. During her time at Ohio State, she wrote for the university's student-run newspaper, The Lantern, and interned at CityScene Media Group in Columbus, Ohio.