The primary difference between Bank 1 and Bank 2 is their position with the engine’s first cylinder. Bank 1 holds cylinder #1, and Bank 2 is located directly across.
So now you know that there is an issue with your car having an oxygen (O2) sensor problem. But hold on, your vehicle has more than one O2 sensor. Bank 1, Bank 2, Sensor 1, Sensor 2… Which one is it again?
Luckily, the exact oxygen sensor is not as difficult to find as one might think. In this article, we’ll explain the methods used to distinguish between a bank 1 sensor versus a bank 2 sensor as well as upstream versus downstream O2 sensor placement.
Bank 1 vs Bank 2
There are two specific designations used when outlining the location of a particular oxygen sensor. The first has to do with which bank of the engine the O2 sensor is associated with.
In the case of “V” configuration engines, one particular side of the “V” will be referred to as bank 1, while the other will be referred to as bank 2.
To correctly number the engine banks, one needs to ascertain the location of the #1 cylinder. For instance, in most V8s, bank 1 contains cylinders #1, #3, #5, and #7 while bank 2 contains #2, #4, #6, and #8.
In any case, the term “bank 1” indicates the first cylinder will always correspond to the engine bank that contains the first cylinder.
Some engines provide labels on their blocks that give information about the bank orientation for the engine. In other cases, you may need to access the specific service manual for your vehicle to try and differentiate one engine bank from the other.
Upstream vs Downstream
The second method of indicating the location of a sensor with a vehicle’s O2 sensor is its position: upstream or downstream of the exhaust.
Typically, a vehicle’s exhaust will be equipped with two oxygen sensors for each engine bank. One sensor is placed upstream from the exhaust inlet while the other is positioned downstream from the vehicle’s exhaust catalyst.
To explain concisely, the phrases upstream and downstream refer to the O2 sensor’s relative position to the vehicle’s exhaust catalyst for the flow of exhaust. This is best explained by imagining the exhaust of a vehicle as a creek that flows.
The catalytic converter in a vehicle would be positioned after the O2 sensor of a certain engine bank, which is upstream, and before the O2 sensor of the same bank, which is downstream.
Oxygen Sensor Locations
Bank 1 Sensor 1 Location
The O2 sensor (bank 1, sensor 1) of a vehicle will be fitted such that it is forward of the catalytic converter on the bank aligned with the first cylinder of the engine.
This is because bank 1 signifies an engine’s #1 cylinder, while sensor 1 denotes an upstream position.
Bank 1 Sensor 2 Location
A vehicle’s O2 sensor will be found on (bank 1, sensor 2) downstream of the catalytic converter on the bank corresponding to the first cylinder of the engine.
This is because bank 1 signifies an engine’s #1 cylinder, while sensor 2 indicates a downstream position.
Bank 2 Sensor 1 Location
In a vehicle, the (bank 2, sensor 1) O2 sensor will be located in front of or upstream of the Catalytic converter on the bank that corresponds to the second cylinder of the engine.
This information can be derived from the fact that bank 2 is representative of an engine’s #2 cylinder, while sensor 1 is an upstream designation.
Bank 2 Sensor 2 Location
In a vehicle, the (bank 2, sensor and 2) O2 sensor will be located at the rear of or downstream from the catalytic converter on the bank, referring to the second cylinder of the engine.
This information can be derived from the fact that bank 2 is representative of an engine’s #2 cylinder, while sensor 2 is a downstream designation.
How can I find the cylinder numbers?
You can locate the number of cylinders in different ways. Certain vehicle models have the cylinder numbers stamped visibly on the crankcase cover.
If you have ignition cables, there are also often numbers on those, but it is not a recommended method because those cables are prone to being swapped.
The most secure way is to check the crankcase cover or identify the cylinder order in your service manual or repair manual. You could also contact your authorized dealer to see if they have any information available.
In addition, you could try searching online using your engine code along with firing order or cylinder order.
Can There Be More Than Two Oxygen Sensors Per Side?
Typically, the maximum number of oxygen sensors per bank is two. This is because all engines utilize an O2 sensor for each end of the catalyst installed. Thus, “V” configuration engines have a total of four oxygen sensors to monitor engine functions and report to the vehicle PCM/ECM.
On another note, two is the most commonly used number of oxygen sensors for four-cylinder engines. The explanation for this is that this type of engine has only one bank of cylinders that contains a single catalyst. This means that one upstream and one upstream O2 sensor would suffice.
How to Locate an O2 Sensor Via OBD2 Scan Tool
An OBD2 scan tool can help find the location of a given sensor. This is simple to accomplish and takes a matter of minutes to complete.
Furthermore, this method of locating an O2 sensor is quite reliable; it optimally saves you money and effort by eliminating the possibility of installing the correct sensor on the wrong O2 sensor.
Before attempting to find a specific O2 sensor, ensure that the ignition of your automobile is set to the ON position and that the OBD2 scan tool is plugged into its corresponding port. You can then start the process of removing each O2 sensor individually while monitoring the active codes on your scan tool.
An O2 sensor that is disconnected should generate an ‘open’ code or a ‘voltage-high’ code along with a sensor designation. This code should disappear after the sensor is reconnected and the ignition is cycled.
From this point onwards, you can continue to disconnect each sensor, one at a time, until you identify the required sensor.