Which O2 lead goes where?

I've been getting the check engine light on my '96 355. It is various oxygen sensor codes (low voltage or no activity). I replaced one of the sensors and swapped some around to see if it is the sensor or the wiring (a previous owner did some creative rewiring here and there).

Just to see if maybe the pre and post O2 sensors were plugged into the wrong connectors (ie, swapped around) I swapped them around on bank 1, cleared the codes, and went for a drive. The check engine light came on after about 20 minutes and I had codes again; some different ones, but all O2 sensor related. The strange thing is that the car ran fine both before and after swapping the leads around. Idles smoothly, revved perfectly. Didn't seem to matter which O2 sensor lead went where other than error codes.

So how do I tell which O2 sensor plugs into which connector on the main harness? There is no blue band that some people talk about and the connectors seem identical? Is there an easy way to tell?

thank you,

Rick

 

You guys did read the part where I said there is no blue band on mine, right?

Checking from the ECU pins did come to mind. Maybe that is the only choice now.

thanks,

Rick