Po700 means a trouble code has been set in your Transmission Control Module, and probably has to do with an internal error in the TCM itself. You need to have the TCM read to see if there are other codes in there.
The TCM is the heart of the electronic control system and relies on information from various direct and indirect inputs (sensors, switches, etc.) to determine driver demand and vehicle operating conditions. With this information, the TCM can calculate and perform timely and quality shifts through various output or control devices (solenoid pack, transmission control relay, etc.).
The TCM also performs certain self-diagnostic functions and provides comprehensive information (sensor data, DTC's, etc.) which is helpful in proper diagnosis and repair. This information can be viewed with the DRB scan tool.
it's generally not a good idea to ignore your check engine light.
Code 645 relates to a problem in the AC clutch relay circuit, so that's something else.