this can often be one of the relays in the blue relay box (my bet is the "main C" relay). there may also be a "spare" relay if this is not a dual fuel truck, that in the legend inside the relay box lid is labeled "fuel pump" and since there is no fuel pump for an LPG system, you can move the last relay (fuel pump) over to the main C position and fix this for very little effort or money. you can use the old relay as a sample to get a new one from an auto parts store if you can not get to a Mitsubishi or Caterpillar or Nissan forklift dealer. I 1/2 way think the PN from NAPA is a806, but could not promise that to be the case.