What should you do after turning on electrical power to test the low pressure warning signal?

The correct action to take after turning on the electrical power to test the low pressure warning signal is to step on and off the brake pedal. This is important because engaging the brake system allows air to flow through the system, which helps to accurately test the functionality of the low pressure warning signal. When the brakes are applied, the air pressure in the system drops, and if the warning signal is working correctly, it should illuminate when the pressure falls below a certain threshold.

The other options do not effectively test the low pressure warning signal. Filling the air tank with air does not create the conditions necessary for the warning signal to activate, as it would keep the pressure higher. Starting the engine might also not cause a change in the pressure that would trigger the warning signal unless there is a drop in air pressure while the engine is operating. Checking engine oil levels is unrelated to the functioning of the air brake system and would not assist in testing the low pressure warning signal.