Monday morning. 8:12 AM. A 2019 Chevrolet Equinox rolls into Bay 3 with a check engine light and a customer who needs the car back before noon.
The technician grabs the scan tool, navigates through a cascade of menus—System Selection, Control Unit, Read Fault Codes—and watches as the screen populates: 15 codes. P0300. P0420. P0171. P0174. U0100. C0561. The list scrolls on. Some are active. Some are history. Some are phantom codes triggered by a low battery three months ago. Some are the actual problem. Which ones? Nobody knows yet.