The car will not start and the engine light does not turn on at all?

The car will not start at all, it cranks but there is no spark or fuel pump engaging. Everything on the car is in good condition but the ecu is shorting out because the battery feed to the ecu goes from 12.8v to 2v when all the plugs are plugged into the ecu. None of the wires are shorting out until they are plugged into the ecu. There were three pins in the ECU that were burnt out but I resoldered them and it still won’t start or give an engine light at ignition. Do I need to replace the ECU, if so what's the process with things like immobilizer and key to work with new ECU.