What was found
LG AIR/GROUND FAIL, DID NOT RETRACT ON TAKEOFF AFTER 400FT. PLACED AIRCRAFT ON JACKS TO TROUBLESHOOT FAULT, WHILE AIRCRAFT IS OFF GROUND, AIRCRAFT GOES INTO ESSENTIAL MODE WITH "ELEC ESS ERF FAIL" MESSAGE ON EICAS. PERFORMED FIM, FAILURE ON ESSENTIAL BUS CONTACTOR #2. FOUND NO GROUND SIGNAL AT PIN E ON CONNECTOR J1150. TIGHTENED LOOSE CANNON PLUG ON P0775 OPS CHECK LANDING GEAR SWING SAT.