NOxSensor

NOx Sensor

Group: Emissions and exhaust

The NOx sensor measures nitrogen oxides in the exhaust. On modern diesels it controls AdBlue (DEF) dosing in the SCR system; on some gasoline engines, it monitors the NOx storage catalyst.

Contents

What does the NOx sensor do?

It has a small control unit of its own that heats the ceramic element and sends the reading to the computer over the CAN network. Systems with AdBlue usually have two: one before the SCR catalyst, to know how much NOx the engine produces, and one after it, to check that the catalyst is removing it. If efficiency is low, the computer warns the driver and, on many vehicles, limits restarting after a certain distance.

Where is it located?

In the exhaust pipe, before and after the SCR catalyst. Its electronic module is mounted nearby, under the vehicle.

Signal type and reference values

  • 12 V supply, ground and two CAN network wires.
  • Reading in parts per million (ppm), visible on the scan tool.
  • It must reach operating temperature before it reports a value (the scan tool shows whether it is ready).

Values are a guide only: always check the manufacturer's data for your engine.

Symptoms of a bad NOx sensor

  • Check engine light and an emissions or AdBlue/DEF warning.
  • A countdown in kilometers or miles until the engine will no longer start.
  • Abnormal AdBlue consumption.
  • Codes P2200 to P2209 and P20EE.

How to test the NOx sensor step by step

  1. With the scan tool, check that the sensor communicates and reaches ready status with the engine warm.
  2. Compare the NOx readings before and after the SCR catalyst: the downstream value should be much lower.
  3. Check the AdBlue quality (urea concentration) and the AdBlue injector before replacing the sensor.
  4. Check power and the CAN lines at its module connector.

Trouble codes related to the NOx sensor

Frequently asked questions about the NOx sensor

Why is the NOx sensor so expensive?
Because it includes a heated ceramic element and its own electronic module with CAN communication.
Can a bad NOx sensor stop the vehicle from starting?
On many Euro 6 diesels, yes: after a certain distance with the fault, the system blocks restarting to comply with emissions rules. In other markets the system may limit speed or power instead.

Updated September 25, 2026. Values are a guide only: always confirm them in the service manual for your engine.

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