Sorry not with you Mark. I must be honest I called in on the garage and took a quick 20 mins look to see if I could shed any light on the problem. I will see if I can get it into my workshop to take a better look and spend some time on it. It just I don't ever remember seeing a negative voltage down the signal wire of the O2 sensor and wondered if other have come across it. I did check voltage drop across the lambda earth cable and it was less than 20mV so not expecting a problem there.