Vehicle OBD diagnostic line:
Often referred to as on-board diagnostic system, the system can monitor the operation status of the engine and the working status of exhaust after-treatment at any time, and will immediately send a warning to feed back to the driver once the situation that may cause excessive emissions is found;
Illustration details:

Common positions of vehicle OBD diagnostic line:

(Illustration)

1 - Above the left foot position, the manual gear is above the clutch

2 - Above the brake

3 - Above the throttle

4 - Below the ashtray in front of the shift lever (some models may need to take out the ashtray first)

5 - Under the passenger compartment

6 - Near or inside the central armrest box

For more information, please see the article on location in Robust Electronic Synthesis:Where is the line interface of OBD diagnostics for many brands of cars?