This happens on a car that I am looking at buying and want to know if it sounds like just the O2 sensor is failing, or if there are bigger problems...

He says when he is cruising at highway speeds 70+ mph the CEL comes on, and ONLY at these speeds does it come on, and it consitantly comes on at these speeds. He read the code as an O2 sensor code.

Does the O2 just need to be replaced or are there A/F or Ignition problems hidden behind this??

Thanks

Matt