My GE PT7800 posted the Key Panel SHORTED message, none of the touch keys or knob are working. I disconnected power to the unit via the breaker for an extend time. After returning power to the unit it asked to set the time, I was able to turn the dial to change the hour but when attempted to press the dial to set there was no response, the touch keys still didn't work, after a few minutes the Key Panel SHORTED message was posted again.

From my research it appears this issue happens often due to loose connections on the control board. I removed the control panel and made sure all the connections where seated well on the control board. Restored power to the unit and am see the same results and Key Panel SHORTED message. Apparently my issues isn't as easy as a loose connection.

My question is how do I go about diagnosing if the issue is with the control board or one of the other boards or possibly a wiring harness.

I check all solder points on the main control board especially where the key panel connection is and there are no visible signs of failures. This would lead me to believe that the key panel (control panel assembly - part# WB56X25674) might be the likely culprit, the only hesitation I have before ordering a new one is the dial assembly (encoder board - part# WB27X32632) is not fully working as well, when you supply power back to the unit the display ask to set the clock, you can turn the dial to change the clock's hour digits but pressing it to save doesn't work and this piece is not tied to the key panel, but it does plug into the same control board. Is it possible the control board is busy processing the key panel failure so it's not responding to other actions. After about 1-2 minutes the Key Panel SHORTED message is posted again on the display.

Excellent suggestion. When I disconnected the keypad from the control and restored power to the unit I was able to use full function of the dial to set the clock and the Key Panel SHORTED message did not appear, of course the keypad didn't work since it was disconnect.
I feel confident that I need to replace the control panel assembly - part# WB56X25674 (key panel).

Thanks for the support, I will post the results here once I get it replaced.