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How to replace a NOx sensor

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The NOx sensor is a crucial part of the emission reduction system. This guide explains how to replace a NOx sensor if it becomes faulty or damaged.
NOx sensor replacement | Masters of Motion
Vehicle Electronics & Engine Management Need to Know Technicians 1 min read

The NOx sensor explained

An Oxides of Nitrogen sensor, or NOx sensor, is placed within diesel engines to monitor the output of oxides of nitrogen. It helps to keep these harmful emissions within regulated limits by triggering the selective catalytic reduction (SCR) system to reduce the amount of nitrogen oxides being emitted from the exhaust.

As an electronic component, a failing sensor can trigger the onboard computer to trigger Limp Home mode or prevent the engine from starting. Manufacturers often prevent their models’ engines from starting if it cannot fulfil emission requirements. 

What to do if a NOx sensor fails?

If a vehicle is affected by the failure of a NOx sensor, there are likely to be some indicative symptoms such as:

  • Check engine light on
  • Engine non-start
  • Increased tail pipe emissions.

These symptoms can occur because the sensor is now giving an inaccurate reading (or none at all) about the nitrogen oxide makeup in the exhaust. This can lead to the SCR system overcompensating the amount of adblue, potentially causing ammonia slip to occur.

Read our guide to what causes NOx sensors to fail.

If these problems occur as the result of a faulty NOx sensor, it’s very important to repair or replace it as soon as possible.

Is it easy to replace a NOx sensor?

A NOx sensor replacement is a complex task. The sensor itself is a delicate component and replacing one is a job that should only be attempted by a professional.

How to replace a NOx sensor: step by step

Pick up some tips on replacing a faulty NOx sensor with this step-by-step guide.

How long does it take to replace a NOx sensor?

A NOx sensor replacement is an important enough task that can’t be hurried - the job must be done correctly and with the resolution of any lingering error codes on the OBD system.

  1. Locate the faulty NOx sensor

    The NOx sensor may be mounted in different locations within different engines but is most commonly found after the SCR catalyst. This allows the SCR controller to check that NOx levels have dropped to the correct amounts. Some vehicles have pre- and post-SCR sensors, although one is more common.

  2. Remove the sensor and module

    Carefully work to remove the two distinct components – the sensor and the module from their housings. You may need to remove other fixtures or fairings that obstruct your ability to remove the components first. Disconnect the electrical connector to fully separate the parts from the car.

  3. Install the new sensor

    First, clean the threads within the exhaust port to assist a smoother installation of the replacement NOx sensor. An M20 x 1.5 HexTap or thread chaser tool should do the job. Thread in the sensor – it can be done almost entirely by hand but check manufacturers’ specifications for the final torque.
  4. Install the new module

    Now you can install the new module component back into place – generally in the same area as other sensor modules are held together. Ensure you’ve got enough space to work in by removing cable mounting from the other leads and pulling back as required.
  5. Re-establish the electrical connection

    Securely re-attach the electrical connection for the NOx sensor, as well as for any others which you might have disconnected to make space within the box. Tie up the cables as neatly as possible so they don’t touch each other, or even themselves.
  6. Check any error codes

    Check error codes using your OBD system. A number of different error codes may have come up because of replacing just the one part, so check those errors don’t come up again and clear the rest if applicable.
  7. Run any relearn procedures and check fuel trim

    After installing, clear any error codes and then check with the diagnostic tool if a NOx sensor calibration is required, and run the routine if present. Print or save the calibration certificate for your own records, and to give to the customer.


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