When does the check engine light go on for a 2003 Pontiac Vibe?

The check engine light on a 2003 Pontiac Vibe (and most other cars) illuminates when the onboard diagnostic system (OBD-II) detects a problem within the engine or emission control system. There's no single event or mileage that triggers it; it's dependent on what the computer detects as a malfunction. These malfunctions can range from minor issues (like a loose gas cap) to serious problems (like a failing catalytic converter).

The light may come on immediately upon detection of a problem, or it might take some time depending on the severity and nature of the fault. Some problems might cause the light to flash, indicating a more serious issue that requires immediate attention.

To know *why* the check engine light is on, you'll need to have the codes read using an OBD-II scanner. Auto parts stores often offer this service for free. The codes will give a more precise indication of the problem.