This is a common fault that happens in 2 guises, either with or without the engine fan coming on constantly when the engines running (ac on or not).
If your engine fan is not on then it is probably the MAP or MAF sensor playing up, if the engine fan is on then it is an electrical problem which Ford have no solution too, I work for Ford and they have spent hours looking over the car with Master techs, replacing MAP/MAF sensors, the Turbo pod, clusters etc etc, to be honest they don't know what causes it and they don't seem bothered to find out either as its a seemingly rare occurance. Funny thing is if you clear the fault code it usually fixes itself, either that or just wait a few days and it will probably be ok for a few months!
Good old ford eh..