White smoke normally indicates that the fuel injected into the cylinder is not being burnt correctly, meaning that the engine or pump timing could be out, or that there's water or petrol in the fuel. It would depend on how much and how bad this smoke is.

The problem is that very often the diagnostic check does not show faults, and as such it is very difficult to identify a fault. I would not recommend replacing the ECU without definitive proof that it is at fault, as it could cost up to £1000. I would ensure that all filters (air and fuel) are clean and serviceable. I do not have specific details of your engine so cannot give advice on how to check the injection timing - you may have to purchase a workshop manual.