# Upstream vs downstream O2 sensor

Source: https://obdlens.com/compare/upstream-vs-downstream-o2-sensor/
Last built: 941ad311c197960f285f

[Diagnostic knowledge](https://obdlens.com/knowledge/)

For exhaust oxygen sensors, Sensor 1 is upstream, closest to the engine; Sensor 2 is downstream, after the catalyst. Confirm the exact sensor layout in the vehicle’s service information.

| Position | Sensor name | Location |
| --- | --- | --- |
| Upstream | Sensor 1 | Closest to the engine |
| Downstream | Sensor 2 | After the catalyst |

Bank 1 is the side containing cylinder 1. Bank 2 is the other side on V or flat engines; inline engines have only Bank 1. For exhaust oxygen sensors, Sensor 1 is upstream, closest to the engine; Sensor 2 is downstream, after the catalyst.

[P0420](https://obdlens.com/codes/p0420/) · [P0430](https://obdlens.com/codes/p0430/) · [P0135](https://obdlens.com/codes/p0135/) · [P0141](https://obdlens.com/codes/p0141/) · [Brand references](https://obdlens.com/brands/) · [Pending, stored and permanent codes](https://obdlens.com/knowledge/pending-stored-permanent-codes/)

## Which sensor is upstream?

Sensor 1 is upstream, closest to the engine.

## Which sensor is downstream?

Sensor 2 is downstream, after the catalyst.

## Does the sensor name identify a failed part?

This entry does not identify a failed part by itself.

Reference-family membership is not proof that a code applies to every listed ECU or vehicle. Runtime-ready definitions are not physical-vehicle validation. Unavailable readings must not be treated as zero. App and hardware sale availability must be checked separately.

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Reference-family membership is not proof that a code applies to every listed ECU or vehicle. Runtime-ready definitions are not physical-vehicle validation. Unavailable readings must not be treated as zero. App and hardware sale availability must be checked separately.