Ford Explorer: Replace O2 sensor on 97 Explorer?

I went to AutoZone today to have them pull the codes from my computer
to find out why the check engine light is staying on (been on for a
few weeks now). We got a 154, 173, and 1151.

154 says "O2 Sensor Circ.,Bank2-Sensor1 No Activity Detected"

173 says "Fuel Trim,Bank2 Malfunction"

1151 says "Lack Of HO2S21 Switches - Sensor Indicates Lean"

Do these three codes indicate that the upstream sensor on the driver's
side needs to be replaced? Has anyone had similar codes and it turned
out to be something other than an O2 sensor? I'm just wondering if I
should bother doing any further diagnosis or if I should just buy the
new sensor ($59 from AutoZone). The truck has 99,900 miles on it but
I've only owned it since 84k so I don't know if they've ever been
replaced.

Thanks for your help,
Dave Earle

Depending on which engine you have, you either have 3 or 4 O2 sensors (OHV
gets 3, SOHC and V8 get 4). If your upstream sensor wires are a different
color than your downstream sensor wire(s) then they probably haven't been
changed. If I remember correctly on my '97 SOHC two were gray and two were
black. I replaced all 4 of mine at about 95K when I got an O2 sensor fault
code. Since most repair shops recommend replacing them every 60K and the
auto parts stores I went to recommend replacing them at 100K I decided to go
ahead and replace all 4 at the same time. I haven't had a problem since. Be
sure and get the OEM version though and not the Generic version. Not only
was the generic version $10 more, but you have to wire in the connector from
your old O2 sensor.

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