Bank 1, on a 4.3 L engine would be on the drivers side.
The coolant temp sensor on a 2003 Chevy S10 V6 is located on the front of the intake manifold. To replace simply unscrew the old sensor and screw on the new one.
After the catalytic converter.
It is threaded into the header pipe directly after the exhaust manifold on the passenger side.
That should be the sensor located after the catalytic converter.
Bank 1 sensor 3 would be the O2 sensor after the catalytic converter.
That would be the O2 sensor after the catalytic converter.
Need more info, is this the 4 cylinder or 6 cylinder engine and which sensor on bank 1.
That should be the O2 sensor just before the catalytic converter.
Passenger side exhaust pipe directly after the exhaust manifold.
Assuming this is a V-6, bank 1 sensor 1 would be the first O2 sensor in the exhaust on the drivers side of the engine.
Downstream ... This is after the Catalyst Converter.
Bank 1 is the drivers side of the engine and bank 2 is the other side. Sensor 1 will be the first sensor that's screewed into the exhaust pipe where it bolts to the exhaust manifold. sensor 2 will be further down the exhaust pipe towards the converter. There are 2 in the pipes on each side of the engine.