Craby is right about the resistance with solder. It usually only happens with bad soldering. Make sure you keep the iron hot and don't heat the solder heat the wire and place the solder on the opposite side and let it flow in. The only time the solder should touch the iron is to tin the tip before you solder.

 

If you form connections properly without damaging the sensor itself (by using a heatsink) and insulate the readings should be unchanged. The change in resistance through soldering is nominal because the tiny globs dissipate very little current as heat.

 

the 02's read .2 to .7 volts. low volts