now that we have some information to go on.....
this truck should have the GE SX controller
code -86 is for accelerator pot values out of range during self test during auto-calibration routine. (during startup it failed the check)

check your accelerator
it either has defective pot or bad wiring associated with that circuit.

A handset or the proper laptop software will be needed to verify and clear codes and check operations

and since i get the impression you did not know what i meant about the 'type of controls' this unit has i suggest you take edwardt's and my advise and call a qualified tech to come out and verify diagnosis and make the needed repairs.