edward t is right!
Check the battery connections,i could bet theres a faulty cable in some element.
Had the same problem last week in a TOYOTA.
The hidraulics are OK?

you say the motors are "all right" but you should give us information on how you know the motors are all right. what tests, and what were the readings and how were they tested, as specific as possible.
If this is a dual drive motor setup, I would also confirm the steering is being properly seen in both controllers.
I would still have the motors rebuilt by someone that rebuilds a lot of forklift traction motors (not someone that only rebuilds really little DC motors [like a starter shop] or big A/C motors mainly), if new or rebuilt controllers are failing.
Some of those Curtis controllers can 'feel' some things in the motors a meter with a weak battery might not pick up upon, that would make them fault out.

Make sure you can hear the electro brake releasing when you select direction. if the brake is faulty, this also brings up a field fault for some reason?

One advice,try to replace motor and if you do that and error is still present electronic unit is problem because you do not have nothing else.Error in your truck tells you that field on motor is bad and you will be sure only if you replace parts. Electronic, motor probably.or wiring.good luck

Hi all,

updated info :

ypmtom 1: When keyswitch turn on, display normal and main contactor closed normal. When twist throttle up , error 3.1 Field short displayed and main contactor opens at the same time

Sypmtom 1: When keyswitch turns on then main contactor closed (closed contact) and then open (open contact) at the moment and error 3.1 field short at the same time

Checked adready the motor & wiring , everything is ok

The motor may have a problem. Remove motor from truck and do a bench test.