Upstream vs downstream O2 sensor
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 · P0430 · P0135 · P0141 · Brand references · Pending, stored and 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.