P0720 can be :
1. transmission connector or conductor plate in the transmission.
2. Bad TCM module.
3. Intermittent wiring.
4. a host of other things ( download the service manual )

#1 THING TO CHECK IS THE BATTERY !!!

I have a $5 cigarette lighter plugin that shows the voltage, but you can use a meter also.
You want to see 12.1 to 12.5 with engine not running and key in accessory position.
You want 13.8 to 14.2 volts with the engine running.
If the voltage fluctuates, double check the battery ground cable connection at the body bolt.

If the battery is old, replace it.