I reference this article from eTrailer: Diagnosing Overload Warning, No Brakes, On a Tekonsha Prodigy P3 Brake Controller | etrailer.com
Sorry, can't help more than that. I have had the overload message but its been very
intermittent for me so I haven't explored further.
It suggests using the P3's diagnostic troubleshooting settings
"...Press the lower button on the right side of the controller, this is the Menu/Options button. Select the Help setting and press the menu button again. Now, select troubleshoot and press the menu button. Now, using the buttons on the left side of the P3 you can toggle through the battery voltage display, stop lamp display, output voltage display and output current (amps) display.
Using these menus you can determine if your problem is on the power wire to the P3, stop light switch wire or output wire."
It talks about testing the brake magnets
at the right of the article. Also, the link above provides other articles that might be of interest.