A flashing light indicates a misfire above a certain percent that will cause cat damage. When I did misfire cals on the prowler we would cause a misfire and monitor the cat temps until they got to a point that would cause damage. The cal would then be set at this misfire percent. My wife's trooper just set a flashing light. Turned out to be the intake gasket. This will be the second one in a year. I monitored the scanner and the front two cylinders( one on each bank) were misfiring every time.