I called the Frigidaire Parts hotline, and the operator told me I needed to replace the Main Control Panel. I didn't feel sure this was the problem so I explored the internet.
I noticed online that AppliancePartsPros.com had a "talk to tech" button, so tried that. The operator seemed very knowledgeable, and suggested replacing the Vent Assembly 1st, and if that didn't work, call back and order the Main Control Panel. She said if the parts didn't work, I could return them for any reason. I didn't have anything to lose, so did this.
I received the part in 2 days! (no shipping charges) Installed it in a couple of minutes, and it worked! So happy I tried her advice!