Just adding on to scott_s post. Dirty air filter, dirty lpg filter in lock off, leaking fuel injector, leaking assistant injector, dirty/bad mass air flow sensor, vacuum leaks, intake leaks, dirty/bad O2 sensor, plugged catalytic converter just to name a few. Alot of things can give this code. But like he said check of its running to rich or to lean first.

Code E-43 air fuel ratio malfunction truck is running lean or rich check alpha reading and see what the computer is doing adding fuel or taking it away check for vacuum leaks check converter pressure check fuel injector good luck.