O2 sensor error code

[alipon@bmw]

Hi guys,Need your help on this...My ride e36 M50b25. Sometime in Jan,08 my fuel comsumption suddenly increase from around 12L/100km to around 16L/100km so i send it to have a diagnosis and found out that there is an error code on my O2 sensor, erase the error but it come back after a short drive. So conclusion by my mech O2 sensor 'koyak' , so change it to a new one which cost me RM800++. Everything went back to normal until last month my FC increase again. So send it for a check again.. Again is the O2 sensor errror code... Try to erase it but the error came back again after a short drive... So my mech supected there might be some leak in the hose underneath the manifold... so check n check, everything seem ok... Finally he decided to change my 'new' O2 sensor to a good condition used O2 sensor the see whether is it my O2 sensor 'koyak 'again and again the error code after a short drive...This time around my mech told me that it might be my ECU... he's willing to lend me a use ECU to try first... but i still haven't send my car for the ECU swap...So guys, anyone of you here have such experience before or anyone know whats the root cause of this issue..Thanks in advance