On your V6 engine :
Bank 1 is the passenger side of the engine
Sensor 1 is in the exhaust pipe ( or exhaust manifold ) before the exhaust enters the catalytic converter ( upstream )
it is located right below the exaust manifold flanges above the catalitic converter
The oxygen sensor for bank 1 sensor 1 for a 2000 mercury mountaineer is on the cylinder #1 side.
On a Ford Ranger ( V6 engine ) Bank 2 is the drivers side of the engine Sensor 1 would be close to the engine before the exhaust enters the catalytic converter
That should be the sensor after the catalytic converter.
Bank # 1 is the passenger side of the engine Bank # 1 , SENSOR # 1 , is close to the engine in the exhaust Bank # 1 , SENSOR # 2 , is in the exhaust after the catalytic converter ( downstream )
On a 2000 Ford Ranger ( 3.0 and 4.0 litre V6 engines ) Bank 2 is the drivers side of the engine Sensor 1 is close to the engine ( it might even be in the outlet of the exhaust manifold ) BEFORE the exhaust enters the catalytic converter
bank 2 sensor 2 is the passenger side, rear sensor, it is located behide the catalytic converter.
Sensor one is in the exhaust manifold on the back of the engine. Sensor two is in the side of the catalytic converter.
Bank 1 is the passenger side of the engine in your 3.0 liter Ford Ranger Sensor 1 ( upstream sensor ) is in the exhaust before it enters the catalytic converter ( either in the exhaust manifold or exhaust pipe )
The P0155 refers to the Oxygen sensor on Bank 2, sensor 1. Bank 2 on your vehicle is on the passenger side of the engine. Sensor 1 is the first oxygen sensor in the exhaust pipe coming from the engine.
Upstream Oxygen Sensor Circuit Slow Response (Bank 1)
It is the sensor in the outlet of the catalytic converter.