Code P0740, Torque converter?

That code doesn't show in the service information for your vehicle but that is actually a common thing in service info. The code basically means there is an electrical fault in the OD solenoid circuit. Either an open or short circuit. To test the circuit, you can test at the solenoid itself. To access it you drain the fluid out of the transmission and remove the valve body cover. The solenoid pack is under it. Unplug the connector on the OD solenoid and measure the resistance between the two terminals. You should see between 2.7 and 3.4 ohms of resistance. If it shows lower the coil is likely shorted if it's higher it is open. In either case you replace the solenoid. For that you remove the retainer and pull the solenoid out. Lube the seals on the replacement with transmission fluid and install it. The code will clear itself as soon as the TCM runs the self-test.
If the solenoid tests okay, then the issue is likely in the wiring. This is a simple system. Battery voltage is on one wire and the other is grounded by the TCM as it is selected. However, Kia doesn't show the actual wiring harness colors, just the simple drawing attached. If it is correct, you should be able to test the solenoid right at the connector Pins 5 and 9 or 10 (split power inputs) should give you the same resistance as the other test without removing the fluid or cover. As they all get the same power feed it is more likely the solenoid or the control side. To test that you need a scan tool to turn it on and test if pin 5 grounds. If yours is somewhat intermittent I would check that connector for corrosion first.