how old is this truck? you are aware that the emission systems components on a lot of trucks have a longer warranty than just 1 year? I can not speak to NACCO products, but I know some other brands do.
that said, I don't have any books on this serial number series, and it is most likely you will be making a call to the local hyster dealer, as they are very close to the chest with the their cards as to how to access the control systems, and this sounds as if the truck is shutting off fuel supply to prevent damage because the controllers input signals are not within the expected ranges some where.