A Nissan Sentra "service engine soon" light can come on for many reasons, including a loosened gas cap. If the "service engine soon" light stays on, it can indicate problems that need to be fixed as soon as possible.
When the "service engine soon" light comes on and flashes, the car should not be driven. Drivers should pull over and have it towed to their nearest mechanic. Continuing to drive the car with the flashing light can result in severe damage. The reason for the "check engine" light can be minor or something very serious, and a mechanic can diagnose the problem.