This site (found with a Google search) may be of some help:
http://autorepair.about.com/cs/troubleshooting/l/bl_obd_main.htmThese codes are only concerned with the upstream sensors so there is no need to even consider changing the downstream sensors. The downstream sensors only have to do with cat effeciency.
It is uncommon for both upstream oxygen sensors to act up at the same time. One of those codes was for high voltage which would be an indication that it is reading a rich mixture. I suppose it's possible that the radiator side sensor is malfunctioning and causing the engine to run rich causing the high voltage reading on the firewall side, but that is just speculation. I would be more suspecious that for some reason the engine isn't running well and that the oxygen sensor readings just reflect that without necessarily being defective.
Good luck with your research, and perhaps someone that has experienced those codes can chime in.
