If the o2 sensor is bad, replace it. If the sensors are activating the check engine light, then the codes should be checked. The catalytic converter could be blocked or the engine may not be running properly.
O2 sensors are always located on the exhaust system.
Most likely there is another issue which may not show up on a PCM code scan. The O2 sensor is reading a lean mixture so, the PCM is giving the engine more fuel. Check for vacuum leaks. The PCM will also richen the mixture to compensate for other problems.
Your Villager has only one O2 sensor located on the front catalytic pipe before the converter. You should be able to look down between the engine and radiator and see it.
I wasn't aware that there was a middle oxygen sensor. I have a 1997 Villager that was vandalized and the catalytic converter and O2 sensors were stolen. From what my mechanic tells me and Auto Zone there are only 2 sensors, one in front of the catalytic converter and one behind it.
mass air flow sensor, O2 sensors, bad fuel pump
You need to check the wiring to the O2.
The check engine light would illuminate and set a code for the bad O2 sensor.
The oxygen sensor for bank 1 sensor 1 for a 2000 mercury mountaineer is on the cylinder #1 side.
There are two oxygen sensors, one in the exhaust near the engine, one in the exhaust pipe after the second converter, roughly under the passenger seat on OBD-II vehicles.
P1131 = Insufficient switching O2 Bank 1 Sensor 1 Sensor/lean. Could be a B1 S1 O2 sensor going bad.
Installing an o2 is very easy. Remove the neg. battery cable. Then disconnect the o2 power connector. Feed the wire through the box end of wrench. Finally, unscrew the o2 sensor.
Try replacing the upstream O2 sensor