What does the engine Immobilizer got to do with replacing the engine?! These clowns here in Nigeria that call themselves mechanics and rewires. Run for your dear life.

The problem with the car is pure electrical. If the electric wouldnt power the relays, sensors and finally ECU, no Jupiter will make that engine to run. If your car security system becomes faulty (which is electrical), your ECU wont run the Injectors, etc. I think 406 ECUs have inbuilt Immobilizer (am not sure). If that is the fault, borrow ECU from another 406 Prestige and try. Or you find the car immobilzer and disconnect it.

It could even be a minor electrical fault, that wont have anything to do with the ECU. Do you have the 406 workshop manual and wiring diagram? If not, try to get one, to save yourself from the "Clowns" that will kill your car, in the name of trial and error. The manual will show you how to troubleshoot your car electrics and the location of electrical components in the car. It wil also tell you where and what to check, replace or do when the car is having various faulty smyptoms. You wont need any rewire again to diagnose your car if you have the car workshop manual.

Ikenna.