Changing the downstream oxygen sensor on my 2002

The 2005 Jeep Grand Cherokee Laredo 3.7L typically has two oxygen (O2) sensors. One is located upstream (before the catalytic converter), often referred to as the pre-cat or upstream sensor, and the other is located downstream (after the catalytic converter), known as the post-cat or downstream sensor. In many cases, the upstream and downstream sensors are not the same and serve different purposes in monitoring and optimizing the air-fuel mixture and emissions.

Here's a link to your firing order,cylinder 1 is bank 1. Specifications Firing Order AutoZone com

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.

P0136 = Oxygen Sensor Circuit Malfunction (Bank1, Sensor2)
P0141 = Oxygen Sensor Heater Circuit Malfunction (Bank1, Sensor2)

CHECK WIRING BEFORE REPLACING SENSORS

If 4 cylinder engine, there are only two O2 Sensors - Sensor 1 is on top, Sensor 2 is at the bottom

If 6 cylinder, there are four (4) O2 sensors (Bank1, Sensor2 is on the firewall side of this engine and is referred to as the 'DOWNSTREAM' sensor (the one AFTER the Catalytic Converter)

Hi, there are 4 sensors on your truck--two upstream and 2 downstream. The upstream sensors are mounted in the exhaust manifolds and the downstream sensors are mounted just behind the catalytic converters. The upstream and downstream sensors are different. Bank 1 is on the driver's side and bank 2 is on the passenger's side. Please let me know if you have more questions, and thanks for using FixYa.

Well upstream from the Catalatic Converter means towards the engine and that would be the location for Sensor #1 on both sides of the engine. And Downstream would be away from the engine past the Catalatic Converter For Sensor #2 on both sides of the engine. Bank 1 or Bank 2 would indicate which side of the engine the trouble is on. ( Bank #1 is the side of the engine that contains #1 Cylinder. Please remember if you are going to change one 02 Sensor then you should change all of them and be sure to clear all codes after all repairs are finished. that will turn the check engine light off. Good Luck and have a good XMASS.

2001 Audi A6
EOBD code: P0420
Fault location: Catalytic converter system, bank 1 - efficiency below threshold
Probable cause: Catalytic converter, wiring, heated oxygen sensor 2 (downstream)

EOBD code: P0430
Fault location: Catalytic converter system, bank 2 - efficiency below threshold
Probable cause: Catalytic converter, wiring, heated oxygen sensor 2 (downstream)

P1133 HO2S Insufficient Switching - Read Our Article On Oxygen Sensor Codes For Help With This Pontiac Trouble Code (check your oxygen sensor also called O2 sensor)

All the codes you have listed come back and refer to the oxygen sensor. I would say that it is your problem. you vehicle may have 2

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1/1 is upstream sensor on the number 1 cylinder side, 1/2 is the downstream of the number 1 cylinder side, 2/1 is the upstream of the number 2 cylinder side, 2/2 is the downstream of the number 2 cylinder side. Hope this helps.

2002 PCED OBDII-Villager SECTION 1: Description and Operation Catalyst and Exhaust System Overview The catalytic converter and exhaust system work together to control the release of harmful engine exhaust emissions into the atmosphere. The engine exhaust gas consists mainly of nitrogen, carbon dioxide and water vapor. However, it also contains carbon monoxide, oxides of nitrogen, hydrogen, and various unburned hydrocarbons. Carbon monoxide, oxides of nitrogen, and hydrocarbons are major air pollutants, and emission into the atmosphere must be controlled.
The exhaust system consists of an exhaust manifold combined with a pre-catalytic converter, upstream heated oxygen sensor (HO2S11), (located just in front of the pre-catalytic converter), three way catalytic converter (TWC), downstream heated oxygen sensor (HO2S12) (located in front of the TWC), a muffler and an exhaust tailpipe.
Heated Oxygen Sensors The Villager is equipped with both upstream and downstream heated oxygen sensors (HO2S) (Figure 1). The upstream heated oxygen sensor (HO2S11) is used to control engine fuel flow. The sensor detects the concentration of oxygen in the exhaust gas and sends a signal to the powertrain control module (PCM). In a rich condition, the HO2S will output a voltage of 0.6 volts or greater, and in a lean condition a voltage of 0.4 volts or less will be output.
The downstream heated oxygen sensor (HO2S12) is used primarily for monitoring the efficiency of the catalytic converter system. The HO2S12 works in a similar way to the HO2S11 but only indicates to the PCM that the catalyst is functioning properly. In the event that the HO2S11 fails, fuel control will be switched over to the HO2S12 and the malfunction indicator lamp (MIL) will be illuminated. This allows the vehicle to operate with fewer emissions, even with a sensor failure.
Figure 1: Typical Heated Oxygen Sensor (HO2S)