Greetings friend ... what do you mean with what is already armed? Is that the engine weapon from scratch? Well; but you can read the scanner; Search for discarding while you manage to keep it on for you to scan. If it was totally armed; I suggest that you begin by reviewing the terminals of your battery (without sulfate and the appropriate adjustment) Check the ground or mass of the negative pole of the battery. If you use coil (s) or distributor; Check the engine's ignition order (I don't know what engine is your truck) but it passes you a tips. If it is engine in "V"; Locate in the cylinder or spark plug cable close to the Damper; That will be your cilinro n The ignition order is important to start the engine. Check that your spark plugs are suitable for your car or are well settled, their cables are equally important. Check the elèctric connections of your gasoline pump (without sulfates, parties or loose cables) Check that in the motor army will not leave the high pressure hose of gasoline, if you see that it is well locate in your injectors (that they are connected, without parties or loose cables) assumed that they are in good condition (very unlikely they are not impossible!
Check that it does not have an obstruction in the air inlet to the engine (if oxygen is missing it does not make combustion) so you will not start it ....
also check; If this is the case; Your alarm system or short current; It may be that it is blocked or short and does not allow the engine ignicion (that would be with an electroauto). A car not linked for 2 or 3 motifs: lack of fuel, lack of current or spark, and wrong driver .. hehehe ..
I hope my comments help you, not dems consult with a trusted meconnic .. luck and comment what I had when solutions.