Assuming the engine is a 4.3L, bank 1 would be on the drivers side.
behind the cat
Bank one sensor two refers to the oxygen sensor. The bank one sensor two oxygen sensor is on the front side of the engine, after the catalytic converter.
The oxygen sensor's are heated. so I guess that's where you are comming up with the word heated. Bank means- what side of the engine that the oxygen sensor is on. Bank 1 or bank 2
There is a failure in the oxygen sensor heater.
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.
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 )
Bank 1 oxygen sensor is the closest to the engine exhaust manifold. It is usually can seen and replaced if necessary with open hood.
Bank 1 is the passenger side of the engine
In a Ford Expedition : Bank 1 is the passenger side of the engine Sensor 1 would be close to the engine before the exhaust enters the catalytic converter ( upstream sensor )
The bank 1, sensor 3, O2 sensor would be the O2 sensor post cat.
Bank one is the side of the engine with cylinder #1. Sensor 2 is after the catalytic converter.
Your in luck...that should be the oxygen sensor in the exhaust manifold at the front of the engine compartment. Bank 2 is the bank of cylinders that does NOT contain the #1 cylinder. That bank is bank 1. Sensor 1 is the oxygen sensor upstream of the catalytic converter, and sensor 2 is the one after the cat.