Check engine light flashes on and off. What can it be?

A flashing check engine light indicates a serious misfire which is considered a critical failure and the car should not be driven long that way. There are many things that can cause misfires, including defective plugs, wires, coils, sensors, fuel injection problems, or worn and damaged engine components.

STOP DRIVING IT!!! When the check engine light flashes it means you are causing damage to the engine or its components. Start over and check everything you did and then some.