Poor cable connection between upper and lower board
Bad lower board
Bad upper board
Inspect the cable between the upper and lower board for damage, crimps, or tears. Inspect the connection of the cable at the upper and lower board. Replace cable if damaged or replace any poor connections.
Verify LED D25 on the lower board is flashing. If not, test with a known good lower board. Replace lower board if necessary.
Verify LED D25 on the lower board is flashing. If not, test with a known good upper board. Replace the upper board if necessary.