F81 means correct ratio not detected [output too low for gear selected] transmission disabled. you need to check wiring connectors on the pots on the right side of transmission if ok you then need a pressure check done on transmission by a Ford tractor dealer or someone with gauges and knowledge as it may mean clutch packs are worn these can be checked by doing a calibration of gearbox it will be expensive if its clutch pack problems.
enginear, September 2010