When is the correct time to verify that all the instrument lights function properly?

Verifying that all the instrument lights function properly when the key is first turned on and the engine is off is essential for several reasons. At this stage, the vehicle's electrical system is energized, allowing the dashboard indicators, warning lights, and gauges to illuminate. This initial test ensures that all lights are operational and can alert the driver to any potential issues with the vehicle when the engine is started.

Additionally, performing this check at this point allows for easy identification of any burned-out bulbs or malfunctioning indicators without the distractions or complications of driving or engine operation. Since the driver has time to assess the dashboard in a stationary position, it's the optimal moment to ensure proper functionality before beginning a journey.

Checking the lights when cranking the engine, while driving, or after the key is turned off would not allow for a thorough assessment of the instrument lights, as not all systems may activate or be clearly visible in those circumstances.