NOx sensor location in a diesel exhaust system before and after SCR catalyst

Where Is the NOx Sensor Located? Position and Function in Diesel Cars

par Sezgin altınöz le April 14, 2026 Catégories : Héros de l'injecteur

Where Is the NOx Sensor Located in a Diesel Car?

The NOx sensor is installed directly in the exhaust system of modern diesel vehicles. It plays a critical role in monitoring emissions and ensuring compliance with environmental regulations.

Most diesel vehicles are equipped with:

  • one NOx sensor before the SCR catalyst (upstream)
  • one NOx sensor after the SCR catalyst (downstream)

This configuration allows the system to monitor both raw and treated exhaust gases.

A NOx sensor is located in the exhaust system of diesel vehicles, typically before and after the SCR catalyst. It measures nitrogen oxide levels and helps control AdBlue injection and emission reduction.

Diagram showing NOx sensor location in diesel exhaust system before and after SCR catalyst with AdBlue system
NOx sensor locations in a diesel exhaust system: upstream sensor before the SCR catalyst and downstream sensor after the SCR system.

Upstream NOx Sensor (Before SCR)

The upstream NOx sensor is located before the SCR catalyst.

Key functions:

  • measures raw NOx emissions from the engine
  • provides baseline data for emission control
  • helps the ECU calculate AdBlue injection

This sensor is essential for accurate emission management.,

Downstream NOx Sensor (After SCR)

The downstream NOx sensor is positioned after the SCR catalyst.

Key functions:

  • measures treated exhaust gases
  • monitors SCR system efficiency
  • compares values with the upstream sensor

It ensures that the emission reduction process is working properly.

Why Do Diesel Cars Have Two NOx Sensors?

Modern diesel engines use two NOx sensors for precise emission control.

Benefits:

  • real-time comparison of emission values
  • optimized AdBlue dosing
  • faster fault detection
  • compliance with Euro 6 emission standards

Typical Installation Points in the Exhaust System

The NOx sensor is usually installed in these areas:

The sensor is typically screwed into the exhaust pipe and connected to the ECU via wiring

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If your vehicle shows emission errors, a faulty NOx sensor may be the cause.

Next Article

➡ Symptoms of a Faulty NOx Sensor in Diesel Cars