What causes the check eng light not to come on when key is in on position?

If the check engine light does not illuminate when the key is in the "on" position, it may indicate a malfunction in the vehicle's onboard diagnostics system or a blown bulb in the dashboard. Additionally, issues with the vehicle's battery or wiring could prevent power from reaching the instrument cluster. It's also possible that the engine control unit (ECU) is not functioning properly. A diagnostic scan can help identify any underlying issues.