940 Non-Turbo

Hi!

Having problems with O2 sensor. Replaced a used but working unit and still have the check light with 2-1-2 fault code. I know the replacement unit is working because it was replaced earlier for an idling fault but discovered later it was due to something else. So, I kept the old one.

Tested voltage: 0.5V for the green wire that goes to the CPU. Fluctuating between 0.2 to 0.4V for the sensor end terminal (black wire) when car is idling. My sensor has three wires - 2 white and 1 black.

Read here there's supposed to be a ground terminal for the exhaust. Where exactly is this ground point. Engine ground with the web-like cable from the valve cover to the firewall is okay. So where can I check the exhaust ground or what else to check?

Thanks.

Laozong