I am a student at a vo tech in Missouri and i had to work on a teachers f 150 that the check engine light came on after driving about 20 minutes it turned out to be the egr valve was not getting vacuum at cruising speed when it opened due to a carbon build up in the intake where the egr bolts on. again i do not know what a code 51 is but it may not be the problem what if that code was just there and never got erased and a new problem is causing the light to come on and the computer just does not store the new problem code, like this truck.