How can I fix ECU unit on Citroen C3 Desire (2003 model)?

My ECU unit got waterlogged, as some mechanic in a garage forgot to replace the ECU and battery cover some time ago. Now engine failure symbol did light up and car lost power. RAC man did drain ECU unit and dry it manually and car was fine for a couple of days. Same thing happenend again and this time car had to be taken to garage where they openened ECU unit and dried by radiatior. Car was fine for a couple of days, but lost power again. Do not want to replace ECU unit as very expensive. Any ideas?