Hi Kevin,
I've no experience with Nisan truck but I do know that the Cat trucks have the same engine and I suspect the same engine management system, any way Problem list for E38 is the LPG injector fault when a short circuit of the fuel pressure sensor occurs
When an abnormal rise in fuel pressure in the fuel pipe from the LPG vapo continues for an excessive length of time.
If the wiring is anything like the Cat trucks then the wires may break around and near the injector and sensor but you won't see the break because it happens inside the insulation near the connectors waggling them may produce a reaction like you can rev the engine ,
The other bad news is that even if you cure the fault you will probably need a lap top to clear the code, or you may be able to do the pedal dance to clear the code , have a look through old cat posts it has been listed on several occasions.

Regards Titus

Code E38 is for LPG fuel pressure to high,it either needs adjusted or the regulator needs rebuilt.