the one constant i've noticed with impco is every time they update their versions of diagnostic software they at some point change their canbus adapter and also their passcode for the "Tier" versions. The problem is they don't send out bulletins telling dealers this right away.
So with that said you better know what Version of firmware is on your truck before you hook up or you won't get anywhere. It simply will not connect.
I ran into this problem back when Cat had Tier 3 out and was in transition to the new Tier 4 trucks, it seems some information was not publically available and even some dealers were not updated unless they asked for it. The canbus adapter and software had to be obtained from Impco directly and after a couple of attempts and 2 different versions of the Tier 3 software we finally got onboard (connected) and could run diagnostics on the late Tier 3 truck i was working on. That version of software we had to use was not the same one you could get off their site.

And to add here... make sure the truck your working on actually uses the Impco software for diagnostics in the first place. If it does not have the spectrum engine or fuel system on the truck your wasting your time and most likely need the correct diagnostic software that Hyster provides for that truck.

Like what has been mentioned previously in alot of cases the handheld OBDII scanner might connect if the harness on the truck has that connector on it, cat had that option on their trucks and i'm sure other mfg's did too. I don't know for sure about the hyster trucks though.

But if you have the right info you most certainly can diagnose a truck without the laptop and even make the repairs, it's just in some cases you may need the laptop to reset codes for those that get hardset and can only be removed by the laptop program.
But then again disconnecting the battery for a period of time generally clears any codes stored in the ECU.