Similar problem. Tyrned out it was a hose that was slightly dry rotted and the cracks were keeping the system from establishing a good vacuum. Check for any hoses that might be cracked.
Its located on your differential
4
Passenger ===================================================== Actually , bank 2 is the drivers side of the engine in a Mercury Mountaineer
There are a total of ( 4 oxygen sensors ) according to the engine wiring schematic for the 1997 Mercury Mountaineer in my Haynes repair manual ( 2 oxygen sensors are " upstream " sensors used by the PCM to adjust the air / fuel ratio of the engine ) ( 2 oxygen sensors are " downstream " sensors / catalyst monitors )
They are located in the door latch on the door
On the exhaust pipe, lower, possibly near the catalytic converter.
For a 1997 Mercury Mountaineer ( 5.0 liter V8 ) ( there are a total of 4 oxygen sensors , 2 sensors that are used with the PCM to adjust the fuel ratio that are nearest the engine on both sides , and 2 sensors that are also called catalyst monitors because they are in the exhaust after the catalytic converters ( downstream sensors ) On the passenger side of the engine in a Mercury Mountaineer ( Bank 1 ) Sensor 1 ( upstream sensor ) and Sensor 2 ( downstream sensor ) On the drivers side ( Bank 2 ) Sensor 1 ( upstream sensor ) and Sensor 2 ( downstream sensor )
Bank 1 is the passenger side of the engine in your Mercury Mountaineer Sensor 1 would be close to the engine before the exhaust enters the catalytic converter ( upstream sensor )
I'm not a mechanic / technician but I noticed in the power distribution box in the engine compartment of a 1997 Mercury Mountaineer that mini - fuse # 11 ( 20 amp ) is for the HEGO system ( heated exhaust gas oxygen ) sensors The PD BOX is " live " - it is recommended to disconnect your battery cables before servicing anything in the PD BOX
I've had that happen in my 2002 mountaineer. what I've found is that if you spray the door latches with wd40 it will clean off the sensors and the door ajar light should go out. but make sure all the doors are shut.
Don't know what your original reading was, but the O2 sensors before the cats moniter how the engine is running and the O2 sensors after the cats moniter how the cats are performing.
you have 4 o2 sensors on the 2002 mountaineer. if you get under the car and look at the exhaust pipe where it goes up either side of the engine you will see it bolted into the exhaust, it has wires coming from it. they are the front 2, the rear 2 are about 2 feet farther to the rear (easier to see)