O2 sensors - how many?
 

Forgive me if you saw this post in another forum far, far away, but I'm really perplexed by this subject that I thought was pretty clear to me:

About 6 months ago I was getting a P0155 DTC, so I ordered the rear O2 sensor and installed it - easy! BIG performance change!

RecentlyI was seeinggetting P0131 and P0137 pop up after pushing it, up a hill...I thought that there were only two sensors on this vehicle: 1) on exhaust manifold (I can see it with a mirror between engine and firewall and 2) just after the Cat Converter (the sensor I replaced).

I took it to repair shop, told them I couldn't reach that forward sensor - please replace. When I picked it up, got sob story that BOTH Bank 1 O2 sensors (one at $175 and the other $199 + labor ...sheesh, the rear one I replaced only cost $32!) had been replaced! Shop says I have a minor coolant leak (possible for this engine - 70K miles although haven't seen fluid level receding yet) that "probably wiped out BOTH the sensors on this side" - said they had to replace 2 of the four front sensors and of course wanted to do the Intake Manifold Gasket job for another $900, which I declined, for now.

So ...how many O2 sensors ARE THERE on this thing? I swear the shop manual I purchased when van was new only referenced two - the ones I mentioned earlier. So what kind of B.S. am I getting from these guys, if any?