Volvo Repair: O2 sensor test, o2 sensors, air leaks

Question
Patrick,
My 98 v70 turbo (98,000) has a check engine lite.  Fault code was P0171.  Your suggestion was check for air leaks in the lines, elbows.  I sprayed 2+2 on all the lines and there was no change in idle.  No change in idle when moving the lines and elbows around either. To me that rules out air leak.
My guess is next to check the o2 sensors.  I assume it is done with a multi meter.  If that is so, how is the test done?  If either needs to be replaced, what will do the job cheapest?  Is there a sensor cross refrenced from an American car that fits?  Usally they are cheaper than ones for cars from overseas.
Thanks,
Eric  

Answer
Hello Eric,  After-market sensors are ok provided you use Bosch,sorry there no Domestic that inter-changes.From under the car,connect to the black wire and one of the white wires,you'll know if your on the correct one because the meter will read 0.54 v - 0.60 vthe higher the # the 'richer' the mix. They don't star to work till they get to 600 degrees.the # should flux high to low in no more that 4/6 sec.Thank you and good luck!  PKD