Car is a '99 v70 standard with 5spd transmission.

My issue is this: I have tried 3 different oxygen sensors in the precat (front) position, including one (non-universal) OE type. Its not that they don't work well, it's that the car isn't seeing them at all. When I access the sensor readouts via an OBDII scanner, the Sensor 1 readout is showing 0.5 volts no matter which sensor I plug in there. The rear (sensor 2) is still working, and varying voltage as it should. With Sensor 2 unplugged, the readout is 0 volts. So my question is, has anyone had the harness to one of the oxygen sensors go dead. My other candidates are possibly a bad relay or bad ECU programming. The failure to read anything other than 0.5v on sensor one started shortly after the previous owner had the dealer perform a free ECU software update (I think related to the ETM).

Please give any help/suggestions you can. I have two other cars so I haven't really addressed the problem, but with gas prices where they are I'd like to put some miles on the Volvo. I'm pretty much convinced that the problem is not with the O2 sensors - after all they wouldn't all show an identical voltage even if they all failed.

 

I'm really going crazy trying to figure out why the ECU reports a 0.5v signal for Sensor 1 to my OBDII PC-based scanner no matter what I do. I have watched graphs over time with 3 different sensors plugged in and get a constant .5 volts every time. Would a bad harness or ECU be the most likely explanations? Or maybe it's just a software issue in the ECU? Thanks in advance for any insight.