Abstract: A control box (11) for a dual-fuel internal-combustion engine (6) is actuated automatically by an oil-pressure switch (16) when activated by a manual switch (30) that directs power from a battery (29) or other engine-electrical component to a current distributor such as a 24-volt bus (49) for operating control-box components that include an under-current sensor (36) for preventing use of gaseous fuel in addition to liquid fuel under no-load and low-load conditions, an over-current sensor (39) for preventing use of gaseous fuel in addition to liquid fuel under excessive-load conditions, an intake-air sensor (17) for preventing use of gaseous fuel with liquid fuel when there is inadequate supply of air, a liquid-fuel-supply sensor (22) for preventing use of gaseous fuel with liquid fuel when there is inadequate supply of liquid fuel, and an exhaust-gas-temperature monitor (23) for shutting down dual-fuel mode when exhaust heat is excessive. Red LED lights (41, 42, 43, 44, 45, 46) warn of system faults.