Your On/Off switch is not providing 12V to the motherboard.
Unscrew right dide of APu and move it away from white wires behind the On/Off switch to see if the short is gone
This code appears when the control system is in engine run mode expects to read voltage in the 8D circuit after the Run Relay (ON/OFF) contacts but the voltage is not present for 5 seconds. Turning APU engine switch off will not generate this alarm unless the engine is running at the same time.
1. Verify the APU engine switch is on.
2. Verify voltage is available on 8X at J15 pin B4. (12 V coming to the switch and from the switch (turned ON) to the mother board).
3. Check connections in the option plug (J4). Verify it not loose, unplugged or missing. This includes the DPF option harness.
4. Verify the DPF option switch is in the on (center) position.
5. Determine if the optional DPF system has an alarm. Fault Relay (FR) is open.
6. Run Relay contacts did not close. Replace interface Board.