Final answer:
The diagnostic trouble codes that can indicate a loose or defective gas cap in a vehicle are P0455 and P0442, which fall under the Evaporative Emission Control system.
Step-by-step explanation:
The diagnostic trouble codes (DTC) that could indicate a loose or defective gas cap are the codes that fall under the Evaporative Emission Control (EVAP) system. One such code is P0455.
This code indicates a large leak in the EVAP system, which can be caused by a loose or defective gas cap.
Another DTC that may point to a gas cap issue is P0442. This code indicates a small EVAP system leak, which again can be caused by a loose or defective gas cap.
These codes are standardized and used by vehicle manufacturers to identify specific issues. By understanding and interpreting these codes, mechanics can diagnose and fix problems with the vehicle.