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Bank 1, Sensor 1? Upstream or Downstream?

Oxygen Sensor

So you’re cruising along on a sunny day and all of the sudden that little yellow engine light pops up on your instrument cluster. You didn’t notice anything funny going on, so you go home and pull out your code reader, and you’re greeted with a trouble code: P0130 Heated Oxygen Sensor Bank 1, Sensor 1 Circuit Malfunction.

You have your DMM ready, and you’re ready to dive in and test the circuit. But, there are four oxygen sensors on this vehicle. Which sensor is it? Lucky for us, there are a few standards in the industry that can help us avoid diagnostic vertigo.

It is important to note that when ordering a part that is labeled left or right, there are standards across the industry as well. The left and right side of the vehicle are always defined as being viewed while sitting in the driver’s seat. The left and right of the engine are always going to be viewed while standing at the rear of the engine and looking towards the front.

Bank 1, Bank 2

In the automotive world, Bank 1 refers to the number one cylinder bank, which means it is the bank where the number one cylinder is located. On an inline four cylinder engine, this is the only cylinder bank. On an inline six cylinder engine, you may see two banks being represented in the scan tool data, when in theory there is still only one actual cylinder bank.

A good example of this is the BMW M54 engine, where two separate exhaust manifolds are used in a single cylinder bank layout. On the M54, the front three cylinders contain the number one cylinder, so this is referred to as bank 1. Likewise, the rear three cylinders are referred to as Bank 2. On engines with opposed cylinder banks, such as a V8 engine, the rule is the same, being that bank one is always the bank containing the number one cylinder. You will need to consult the service manual to find out the cylinder layout for each particular engine.

Sensor 1, Sensor 2

As far as oxygen sensor location goes, things are quite simple:

Sensor 1 is the upstream oxygen sensor. It is the sensor that measures the oxygen content in the exhaust, providing an input to the computer, which determines how to adjust the air/fuel ratio. Sensor 1 is the sensor closest to the engine.

Sensor 2 is the downstream oxygen sensor. It is always going to be located after the catalytic converter. Its job is to monitor the oxygen content exiting the catalytic converter in order to determine if it is operating efficiently. In some cases, there will be a Sensor 3. Sensor 3 is found on some Ultra Low Emissions Vehicles, and will be located after the post-catalytic converter.

Component locations can be misleading at times. Knowing where to look can help save both time and money when performing repairs on your vehicle.

If you are looking to purchase an oxygen sensor follow this link and select your vehicle’s Year Make and Model to get the exact fitment for your car:


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