If the check engine is flashing intermittantly, it meamns that you have a misfire or youe engine not burning all of the fuel supplied to the cylinder(s); possible strain on catalytic converter because it cannot handle the high level of unburned hydrocarbons.
Otherwise you may have some intermittant code causing the light to appear for about three driving cycles(a cycle is starting the car, driving it for short while, stopping the car, turning ignition off and closing driver's door)because the problem had disappeared temporarily.