Hi all hope you can help

I have just tryed to fit a V4 prodrive SP Sti ecu into a standard V3 UK2000 and it brings up the engine check light?

We tryed the swap the other way see pic and it dose the same to the prodrive car?

The light comes on and flashes, the car starts and ticks over badly but wont rev.

both cars work fine with there origional ecus so its not a ecu fault.

There is a extra white conector on the prodrive loom see pic it didnt connect to any thing but its not on the standard V4 UK model. what is it?

I have been under the impression that V3-4 ecus swap freely due to the amount you see for sale advertised as fits V3-4.