Contour Enthusiasts Group Archives Forums Modifications Duratec Maintenance Can I use downstream o2's as my upstream ?

I'm going to order some new oxygen sensors for a tuneup.

The circular ones are listed as the rear (or downstream ?) sensors for the cats and the square connectors are the upstream ones for a/f, right ? Thing is, the o2 connectors on my contour for the upstream o2's are circular than square. Yes. Ford, built on a Friday.

Now, if I order the downstream ones (Denso PN: 234-4609) will this hurt my car ? Are the sensors themselves different or are they all the same ?

Thanks.