It's called "Limp home mode". This system detects faults or problems and allows the operator to move truck to safe location or back to repair area. Are there any codes or dash lights flashing on or near the dash where the hour meter is?
What type of application is the truck used in? The truck will do this if the transmission sensor gets wet or fluid is burnt.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.