A check engine light on a Ford Explorer sometimes indicates a simple problem, such as a gas cap that needs tightening, or something more complex and expensive, such as a bad catalytic converter. It often requires a trip to the repair shop to diagnose the problem causing the warning light.
Automobiles have onboard computers that keep track of many different indicators of how the vehicle is operating. A loose gas cap affects the fuel-oxygen mix, which could trigger the warning light. Tighten the cap and watch the light; if this is the issue, the light eventually stops glowing.
In some vehicles, getting moisture where it does not belong affects the check engine light. About.com says some Ford trucks trigger the light any time it rains. Water drips onto spark plug wires and runs to the head where it causes an occasional short. Getting an engine wet while at the car wash is also known to trigger the light.
Spark plug wires deteriorate over time, developing tiny cracks that allow electrical leaks. The slight misfire causes the light to glow on the dash. Replacing shabby-looking wires often solves the problem.
Low-octane fuel is another reason for the indicator to light. The low octane also causes misfires, which the computer detects. Switching to a midgrade fuel often stops the issue.