This TV responded to the power button and attempted to turn on but there was no backlight. Checking the signals at the power connector, I measured 19V at the 12V pins and 3.6V at the PS-ON pin but BL-ON wasn’t getting a signal so the main board was suspect.

Checking shopjimmy, they sell an eeprom for the main board with the symptom no backlights. They have a video showing the symptom on a similar model P502UI-B1

Ordered the eeprom but the problem was still there. Luckily, new main boards were available for $30 USD from Electroparts online through their ebay store. I have had a good experience from Electroparts since they had free returns and provided USPS labels.

After main board replacement, the TV turned on with backlight. There were also LEDs on the main board that lit up. A post on badcaps for P502UI-B1 indicates BGA failures on the main processor are the suspect and not the eeprom. Usually with eeprom failures, the tv doesn’t respond to the power button.