P0050 O2 sensor fault code

[cmbfoto]

I have a O2 sensor on my 1999 S80 2.9L that flat lined Bank 2 Position 1, Is this the sensor before the Cat or after the cat? Parts guy says it's after the cat and i say it's before the cat. Upstream or Downstream sensor??

Thanks

[Vova585]

Position 1- prior to catalytic converter.
Position 2- after catalytic converter.

2 ways to approach this issue. 1st- to buy quality sensor from reputable source and manufacturer. Bosch, Denso. Can buy it from dealer, fcp euro, ipd usa. Replace the sensor and hope all is good. 2nd way-, look at the wires going to the sensor. Usually there will be 4 wires. Black-signal wire. Grey- ground. 2 white wires will be 12v+ and 12v-. Your code identifies problem with heater element of the sensor. Disconnect sensor from the harness. At the sensor side connector(female) with t-pins or small probes check resistance between 2 white wires. It should be around 6 ohms usually, but can be around 2-20 depending. If you are getting "OL" reading- heater circut is gone and you can proceed to sensor replacement. If you are getting good measurements at the sensor, take a test light and check if you are getting power and ground at the sire of engine harness corresponding to 2 white wires at the sensor. If not- obtain electric diagram for the car(vida, alldata, Mitchell) and start tracing intermediate connectors for a broken wire.