WHAT IS THE BUS?
Year, make, model, engine, transmission, front/rear/conventional engine location. These things matter as there are often common failure modes in some configurations.
PULL THE CODES address any code items relevant to the current problem of no start.
Check battery voltage (ALWAYS first step) In a 12 volt system, 12 volts is 1/2 charged.
Inspect all wiring and connections (not just the big wires ALL of them) at the batteries.
Inspect all wiring and connections AT the ignition switch
Inspect all wiring and connections AT the starter and solenoid.
Check for ignition switch START signal at the starter solenoid. Keep hands clear of rotating parts and trust the person hitting the ignition while you're checking for solenoid activation voltage.
--if no START signal present at the starter solenoid use an ignition bypass cable or fabricate a jumper wire and present battery voltage to the starter solenoid (observing all safety precautions like vehicle in PARK or NEUTRAL, EMERG BRAKE SET, wheels CHOCKED, safety observer present).
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We've done so much, for so long, with so little, we now do the impossible, overnight, with nothing. US Marines -- 6531, 3521. . . .Ret ASE brakes & elect. Ret (auto and aviation mech). Extra Class HAM, NAUI/PADI OpenWater diver