Citroen Saxo - ECU - grapsy

My 2000 saxo engine light will not switch off, and fan kicks in even if i dont start the engine, been told it is an ECU problem , what are they like to change and what cost ?

Citroen Saxo - ECU - Terminator-17

Easy to change - if virginized - but several hundred pounds (£600 for a new, security coded one at the dealers) but if it's the usual coolant sensor circuit problem, the new one will be destroyed instantly too.

What codes have been found? What testing has been done and what exactly is so wrong with your ECU that it needs a new one?