Where is the downstream O2 sensor on a 2011GMC Terrain 4cyl?

`look under vehicle at exhaust the tube looking part with a connector thats after the cat converter going towards tailpipe is down stream o2 sensor

Which one? Upstream, downstream, left or right cylinder bank? (if V6)

You have at least two, possibly four.

If you have dual exhaust, so two catalytic converters on the car, you have 4 oxygen sensors: one for each bank of the engine and one behind each cat. An oxygen sensor after the cat is called a downstream O2 sensor, and is solely for monitoring the efficiency of the catalytic converter.
If you have single exhaust and one cat, you will have this one downstream O2 sensor behind the cat, and then you may have one or two "upstream" oxygen sensors. The upstream sensors are in the exhaust, close to the engine. If you have one for each bank of the engine, these upstream sensors may be right in each exhaust manifold on the engine, or in the exhaust pipe attached to the manifolds, but still close to the engine.
Some cars have only one sensor for the engine and one for the cat. A 2005 buick century with a 3.1L engine has only two sensors, one for the engine or one upstream sensor, and one downstream sensor.

If your Impala has only one upstream sensor, look on the back side of the engine, down low, after the exhaust pipes from the two sides connect. It should be there, the only thing sticking out of the exhaust that has wires to it. Follow the wires to the pigtail connector, if you are replacing it. Before trying to unscrew it, the exhaust should be warmed up. Not hot as in normal running temperature, but just heated up a bit. Less chance of stripping the threads if it is warm. Just don't get burned. If possible to get a wrench on it-7/8 inch open end. If no room for a wrench, stores sell a special socket to remove oxygen sensors.
The upstream oxygen sensors are what the computer reads to adjust or "trim" the amount of injected fuel for the correct air/fuel ratio.
I didn't expect to run on so long, hope you understand it. If not, or any further question, give a shout back.

Bank 1 o2 sensor is located in the exhaust manifold up by the motor bank 2 is located downstream futher in front of the cat.converter mounted in the exhaust pipe.

Yeah, you could do that. Bank 1 is always the engine side where the number 1 cylinder is located. Sensor 1 is the first sensor in the exhaust system, nearest the engine. Sensor 2 is always located farther downstream in the exhaust.

The downstream sensor will be AFTER the cat. It may be on the top of the exhaust pipe and hard to see. As a rule it is close to the convertor.

If the engine is a V6 (2.7 or 3.5 lit), bank 1 is next to the firewall (right hand) when facing the back of the engine.
If the engine is a 4 cly (I4), there is only one bank and is designated Bank 1 (exhaust manifold).

The catalyst to which the code refers is the UPSTREAM, and there is only ONE upstream cat for any bank.

Clue: Two upstream O2 sensor and two downstream O2 sensors - one before the CAT and one after the CAT. the downstream O2 sensors are identical to their upstream sisters. The only reason for the DOWNstream sensor is to monitor the CAT efficiency (threshold).

The catalyst on either bank is essentially the exhaust manifold which houses the CAT... it is the only CAT to which the code P0421 applies.

Hope that helped

you have 4 o2 sensors and 4 catylitic converters. bank 1 sensors are located on the passenger side exhaust. #1 is before the catylitic converters and #2 is after. bank two, on the drivers side, has the same configuration, the downstream sensor is after the cats.

code p 0421 refers to warm up cat converter bank 1 --- efficiency below threshold ===causes ---cat converter---wiring ---HO2S (heated oxygen sensor )
there are not 4 oxygen sensor as you have been told but just 2 oxygen sensors located after each of the cat converters
The other 2 are heated oxygen sensor which are located in the exhaust manifold or exhaust pipe before the cat converters
heated oxygen sensors do a completely different job to the O2 sensors in that they report to the ECM, the combustion gas mixture so the the ECM can adjust the mixture for max performance
The oxygen sensors (O2) read what is left after the cat converters have done their job and report to the ECM so that it can fine tune the mix
IF you didn't replace the correct cat converter then that will make the code come back
If you just replaced the O2 sensor , well that doesn't even figure in the problem

the o2 sensors are locted in the exhaust system. upstream is before cat converter, downstream is after cat converter.