Why is the check engine light on in the ford f350 diesel?

Quite simply, the check engine light being on in a Ford F-350 diesel is a signal from the cars engine computer that something is wrong. If the light begins flashing, it indicates a more serious problem such as a mIS Fire that can quickly overheat the catalytic converter. Do not drive Your car if the light is flashing. The problem can be anything from a momentary hiccup, something like a loose gas cap or a change in humidity, to the failure of a major component like a bad catalytic converter. The list of things that can trigger the check engine light is pretty lengthy. First things first, if the light comes on, You should first check to see if the gas cap is loose. If the gas cap is loose, tighten it and continue driving. Even after tightening, it will take some time for the light to go off.