1996 Toyota Carib PO773 code

Transmission problem
1996 Toyota Carib 4 cyl Front Wheel Drive Automatic

The car has no symptoms other than the check engine light and remaining last wk. Had car diagnosed by auto-zone and a certified mechanic. Took car to a transmission shop as the code said shift solenoid electric.
The solenoid and wiring/filter/fluid was replaced, this past Tuesday.
The code was cleared.
Since then, everytime the car sits for more than 9 hours, the CEL light comes/stays on again. Took car back to transmission shop 2 days ago, they found nothing. Code cleared again. Next morning after driving for 2 miles and restarting, CEL back on again.
This morning took car to auto-zone for a scan. Same code comes up PO773.
Some say to drive car and eventually the computer will read repair has been done. Auto-zone says maybe the solenoid that was replaced was bad.
Will call transmission shop again tomorrow (warranty 90 days).
Any ideas why this CEL and code returns?
absolutely no symptoms with the car, it runs beautifully. Other than the CEL light on.