Truck won't start
Turn key to start and hear click then buzzing
from control/fuse box next to battery.
Battery test good 12.5 volts. No interior
lights other than ignition light. Headlights
won't come on.
12.6 volts is generic spec for full charge, meter leads across battery posts, everything off. Have you tried battery cables or jump box to see if starter motor will crank the engine? You can leave meter leads across battery posts, have helper turn key to crank, how does the voltage reading react? Does it drop way down or not drop at all? Battery cables -- connections all good? I looked at starter motor diagram, 5.9L engine, gasoline. See the starter relay in diagram, unplug the relay, Use jumper wire between terminals 30 and 87, the starter motor should crank the engine. If you try that, make sure transmission is in park or neutral and parking brake is set. Make sure vehicle can't move. As far as buzzing sound, Have helper put hand on relays in fuse box while you turn key to crank, they should be able to feel vibration at proper relay. Then you'll know what relay, maybe the starter relay?