No comunication with ECM

The check engine light is on, the immobilizer light is flashing, there is no communication with the scan tool. The temperature gauge is inoperative. The car runs sluggishly, no acceleration. I am stuck with this problem because I change the ECM and the mechanic reprogrammed the module; everything was perfect in this moment, but when I turn the ignition off and I tried to turn it on again, the same problem occurred. I do not know what is wrong with my car. I know that it has to be an electrical problem, but I do not know where can I find it. I have read some comments posted in this forum and I found somebody with the same problem six years ago and the expert suggested check the cluster. I think that it is only the part under dashboard that I did not replace because I replaced the ECM and Multiplex. Please I need your help in this case
Note: I have already check the power in the plugs' cluster and it is okay. Also, I checked all fuses and relays and everything looks okay.