NOx sensors and O₂ sensors are not the same component; they differ significantly in terms of function, installation location, and operating principle.

The core differences are as follows:
**Differing Functions**

**O₂ Sensor (Oxygen Sensor):** Primarily used to monitor the *oxygen concentration* in the exhaust gas, helping the Engine Control Unit (ECU) regulate the air-fuel ratio to ensure combustion efficiency and reduce CO and HC emissions.
**NOx Sensor:** Specifically designed to detect the *nitrogen oxide (NOx) content* in the exhaust gas; it is a critical component of Selective Catalytic Reduction (SCR) systems, used to control the urea injection quantity in order to reduce NOx emissions.
**Differing Operating Principles**

The O₂ sensor generates a voltage signal based on the difference in oxygen concentration (e.g., the zirconia type); its output signal typically ranges from 0.1 to 0.9V, providing feedback on whether the air-fuel ratio has deviated from the stoichiometric ideal of 14.7:1.
The NOx sensor, conversely, measures the concentrations of specific nitrogen oxides—such as NO and NO₂—using more complex electrochemical or optical methods; consequently, it features a more intricate structure and entails higher costs.
**Installation Location and System Affiliation**

O₂ sensors are typically installed *both upstream and downstream* of the three-way catalytic converter (the upstream sensor is used for closed-loop control, while the downstream sensor monitors catalytic efficiency).
NOx sensors are predominantly found in the SCR systems of diesel vehicles or high-end gasoline vehicles; they are typically positioned *upstream and/or downstream of the catalytic converter* to monitor the effectiveness of the purification process.
**Scope of Application**

O₂ sensors are a *standard feature* on virtually all vehicles equipped with electronic fuel injection systems.
NOx sensors are primarily found in diesel vehicles—or select high-performance gasoline vehicles—that comply with China IV (National IV) emission standards or higher.