If you provide your or truck model year, I can narrow it down further.
Isuzu Elf NKR NPR 4HL1-1 24V Diesel Engine Control Unit ECU ECD ECM 8973738770 | OE | Front | Screw-In | Male | Blade | Plastic | Optimal Durability isuzu 4hl1-1 ecu pinout
If the ECU has no power supply, check the main relay and fuses related to the ECM/ECU. If you provide your or truck model year,