Upstream vs. Downstream O2 Sensors and How to Find Bank 1 or Bank 2

If you’re wondering how to tell the difference of upstream vs. downstream O2 sensors and how to find bank 1 or bank 2 of your car, check out the tips from our expert mechanic in this article and video.

Video: Difference Between Upstream and Downstream O2 Sensors

Upstream vs. Downstream Oxygen Sensors: How to Tell the Difference

What Is an Upstream or Downstream O2 Sensor?

Upstream and downstream O2 sensors placed next to each other for comparison
Upstream vs. downstream O2 sensor

These oxygen (O2) sensors have aesthetics differences. You won’t be able to tell which belongs upstream or downstream by appearance. The wires, body, and tips of the sensors differ, but the threads, gaskets, and electrical connectors are the same. You could replace an upstream O2 sensor with a downstream O2 sensor and not know.

With most oxygen sensors, you can’t tell the difference just by looking at them, but you can tell which sensor is which by looking at their placement on your vehicle relative to the catalytic converter.

Upstream O2 sensors are located before the catalytic converter, closer to the engine. They see the exhaust gases before they enter the cat converter.

Downstream O2 sensors will be placed in or after the catalytic converter. They monitor the gases leaving the cat, and make sure its working efficiently relative to the upstream O2 sensor’s readings.

How to Replace an Upstream or Downstream O2 Sensor

How to find an upstream or downstream oxygen (O2) sensor on your car or truck

  1. Find Bank 1 or Bank 2 on Your Car

    You’ll need to determine bank 1 vs. bank 2 to find the upstream and downstream O2 sensors. You can tell which bank is which by looking at the firing order of your vehicle’s engine.

    The bank that includes cylinder 1 is bank 1, and the other bank will be bank 2, regardless if if it includes cylinder 2 or not. This is true for V8, V6, and flat engines, like Porsches and Subarus that have individual banks.

    For inline 4 cylinder engines, as long as they have 1 exhaust manifold, like a Jeep 4-liter or 4 cylinder Honda engine, there is only one bank for upstream and downstream with no bank 2.

    This means you could need an upstream or downstream for bank 1 or bank 2.

  2. Find the Upstream or Downstream O2 Sensor

    Once you’ve determined where bank 1 and bank 2 is, find the O2 sensor that needs replacing. Then you can compare it to the new one to confirm you have the right oxygen sensor.

  3. Check and Compare the O2 Sensors Features

    As shown in this article’s video at 2:30, we can confirm which oxygen sensor belongs where by comparing it to the features of the defective one still installed.

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Upstream vs Downstream O2 Sensors - How to Find Bank 1 & 2 - 1A Auto
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Upstream vs Downstream O2 Sensors - How to Find Bank 1 & 2 - 1A Auto
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If you want to know the difference of upstream vs downstream O2 sensors and how to find bank 1 or bank 2, check out these tips from the expert mechanics at 1A Auto
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