Help
 

Bit of help needed guys I've got a 97 uk turbo. Had the cel light come on so put car in diognostic mode with the black and green connectors and got lamda sensor fault so replaced it with a genuine one. Left the battery off over night and all was fine until I went on motoway then cel light come on again but this time when I put it in diognostic mode I carnt read any codes because the cel light is permanently on. I've tried reseting the ecu the manual way but nothing is happening the cel lights just stays on permanently on.
Has any one any ideas on what's wrong with it.

Both green and black are for the ECU reset.
Black only to read codes.