2000 Nissan Frontier Truck won't reverse

Lets check TCM for codes 1st Ensure engine is at normal operating temperature. Turn ignition switch to OFF position and wait 5 seconds. Ensure selector lever is in "P" position. Turn ignition switch to ON position. O/D indicator light should illuminate for about 2 seconds. If light does not illuminate, check for short circuit . Repair as needed. Go to next step. Turn ignition off, then to ACC position. Push shift lock release knob and move selector lever to "D" position. Turn ignition switch to ON position. Depress and hold O/D switch button in OFF position. Turn ignition switch to OFF position, then to ON position. Release O/D switch. After 2 seconds, move shift selector lever to "2" position. Set O/D switch button in ON position. Depress and hold O/D switch button. Move selector lever to "1" position. Release O/D switch button to OFF position. Depress and release O/D switch button to ON position. Depress and release O/D switch button to OFF position. Depress and hold O/D switch button. Depress accelerator pedal to floor and release pedal. Release O/D switch button. Record O/D OFF indicator light flashes. Where LONG light flash appears in flash sequence (1st, 2nd, 3rd etc.) indicates code

I can run this test, but not being a mechanic or car person, I need to know something more basic. What might be the various causes of this failure? And when I go to a mechanic (transmission person?), what are the questions to ask so as not to get ripped off. Thanks so much for your assistance.