still dont know the series of the truck
what are the letters after the 040?
titus is right on any of the GE controllers code 57 is a current sensor low voltage error.
So this is 48v truck?
is this the voltage you read when you put meter across battery?
a fully charged 48v truck will read somewhere around 49 to 51v depending on the condition of the battery. If your just getting 48 then that battery is dead... hence causing this code i'd say ;o)

keep the meter on the battery and activate the hyd pump and stall it by bottoming out a tilt mode and then see what it drops to while holding it, the let it go after about 15 or 20 seconds. It shouldn't drop too far below 48v. If you get a large volt drop of lets say more than 5v difference then you should charge the battery, then re-check it, if still dropping alot check cells and see if there are any bad ones.

just a guess though since i have no accurate information to go on yet