if the fault light is persistant then there is a problem that needs to be corrected. It will not just go away until you fix what is causing it.
First you will need to find out what fault code it is.
There are 2 ways to get the codes...
1- get the SST scanner and read the codes in the ecm or...
2- do it the manual way using the accelerator pedal...

if this truck has the nissan K21/K25 engine you can do the pedal dance to activate the self-diagnosis system, just search the term 'pedal dance' and there are many threads here that explain how to get into the system using the accelerator pedal sequence.
Once you successfully access the system it will flash the codes with the MIL light on the dash.

get the DTC codes and come back here and tell what they are and we can go from there ;o)