E-38 = LPG vaporizer diagnostic result signal
There is a MCFA service gram out on this code
it refers to there being an abnormally high fuel pressure detected by the fuel pressure sensor.
Additional troubleshooting information:
DTC E-38 may be set when the fuel pressure in the fuel piping/hose has been abnormally high for a predetermined amount of time. It may set if the sensing circuit is detected open.

-check the fuel pressure
Note:The fuel pressure it may read higher than 42kpa. The pressure can spike upward when you key on and off multiple times.
After cranking the engine begins or as the engine starts, the fuel pressure should begin to drop to 33kpa or lower depending on the year of the fuel system.
If it remains high then check the actual fuel pressure with a maximum 25psi guage, the guage must read 4 to 4.5psi average. if in excess of 5psi then begin looking for a high pressure leakage from the primary to secondary side of the vaporizer assembly.

I would also check the wire harness at the fuel pressure sensor and make sure none of the wires are broken.

If pressures are within the normal ranage then inspect the sensing voltage (above 5vdc) at the fuel pressure sensor.

2005 thru 2010 truck fuel sensor
At the connector on the fuel pressure sensor terminals
-pin 3 (blue/red wire) key on, 5v, engine running, 5v.
-pin 2 (white-green wire) key on 2.5v, engine running volts vary with LP pressure.
-pin 1 (black/red wire) negative
using ohm meter ohm resistance check between pin1 and pin2 must be between 40.4 to 45ohms.

2004 truck fuel sensor
At the connector on the fuel pressure sensor terminals
-pin 3 (blue/red wire) key on, 5v, engine running, 5v.
-pin 2 (black/red wire) negative
-pin 1 (white-green wire) key on 2.5v, engine running volts vary with LP pressure.
using ohm meter ohm resistance check between pin1 and pin2 must be between 40.4 to 45ohms.

If the voltage is normal then replace the fuel pressure sensor.