NOx OFF — disabling the NOx sensors

NOx OFF disables the NOx sensors and their monitoring in the ECU software. NOx sensors are fitted to petrol engines that run a lean mixture (naturally aspirated VW FSI, BMW N43 and N53, Mercedes CGI) and to diesels with an SCR catalyst or a NOx storage catalyst. The file removes the sensor faults and the checks the ECU runs with them. For tuners: send us the original file and get a ready-to-flash file with corrected checksums, on average in 30–60 minutes.

Where NOx sensors are fitted: petrol and diesel

A NOx sensor is a zirconia ceramic element. It pumps the oxygen out of the gas sample, breaks down the nitrogen oxides and works out their concentration from the pump cell current, measuring oxygen along the way. It runs hot: Continental sensors, for example, operate at 800 °C. The first production NOx sensor (NTK) appeared in the early 2000s on the petrol Volkswagen Lupo 1.4 FSI.

Lean-burn petrol engines

A normal petrol engine runs at λ = 1, and the three-way catalyst takes care of nitrogen oxides. Engines with stratified direct injection go very lean at part load, with an average λ of about 3 to 5. That saves fuel, but with so much excess oxygen the catalyst can no longer reduce NOx. So a NOx storage catalyst is fitted behind it, and a NOx sensor behind the storage catalyst.

Many manufacturers later dropped stratified operation: the real-world fuel saving turned out to be modest and the aftertreatment expensive. Volkswagen switched its FSI engines to a homogeneous mixture with a software update, and the turbocharged TFSI and TSI engines have always run at λ = 1, with no NOx sensor.

Diesels: SCR catalyst and NOx storage catalyst

NOx storage catalysts arrived on light-duty diesels around 2007–2009, and by 2017 most diesel cars had one. With Euro 6d many models switched to SCR with AdBlue or added it alongside the storage catalyst. With a storage catalyst, the NOx sensor sits behind it, as on a petrol engine.

An SCR system usually has two sensors. The first, ahead of the catalyst, shows how much NOx is going into the SCR, and the ECU calculates the AdBlue dose from it. The second, behind the catalyst, shows what is left: the ECU uses it to check how well the system cleans the exhaust and to correct the dose. With this feedback an SCR removes more than 80% of NOx, while dosing purely from calculation is best kept at about 60–70%, otherwise ammonia slip goes up. The second sensor also reads ammonia as NOx, and the ECU has to allow for that.

How the petrol storage catalyst works and what the ECU does

The storage catalyst coating contains barium. On a lean mixture it binds nitrogen oxides as nitrates. After about 1–2 minutes the store is full, and the ECU switches to a rich mixture for 1–2 seconds: the nitrates break down and the released NOx is reduced to nitrogen straight away. Then it goes back to lean, and the cycle repeats.

The NOx sensor behind the storage catalyst tells the ECU how much nitrogen oxide gets through. Once the store is full, NOx starts slipping past, and the sensor sees it. The ECU also uses it to check how well the storage catalyst works.

Sulfur is a problem of its own. It binds in the storage catalyst more firmly than nitrogen and gradually takes it out of action. The ECU runs a burn-off from time to time: several minutes of rich mixture at 650–750 °C. That is why stratified-injection engines are designed for low-sulfur petrol only, and the BMW N53 was never sold in the US and Canada because of the sulfur in local fuel.

Why NOx sensors fail

The sensor lives in hot exhaust. On the BMW N43 a failed NOx sensor is on the list of typical problems, along with injectors and ignition coils, and the petrol storage catalyst also gets poisoned by sulfur over time. On a diesel, a faulty NOx sensor is a common cause of an SCR efficiency fault: Ross-Tech's advice for VAG is to deal with any sensor faults and check the sensor electronics modules first, and only then look at the AdBlue injector, fluid quality and the catalyst itself. When a sensor fails, the ECU logs a fault, turns on the Check Engine light and loses its feedback.

Codes per SAE J2012. Manufacturers may also use their own codes for the sensors, the storage catalyst and the SCR.

What NOx OFF changes in the software

NOx OFF and AdBlue Off are different options. NOx OFF takes the NOx sensors and their monitoring off the ECU. AdBlue Off disables the whole SCR system: dosing, heating, fluid monitoring and the no-start countdown. If a sensor has failed but the SCR is fine, NOx OFF is enough. If the SCR system is being removed altogether, you need AdBlue Off.

On a petrol engine without the sensor, the ECU has nothing to keep track of the storage catalyst with, so the solution for each ECU also takes lean mode into account. If it matters to you how the engine will run at part load, say so in the order. If the main catalyst comes off together with the storage catalyst, add CAT/LAMBDA OFF. Stage 1 can go into the same file.

What the tuner should do

• Before ordering, check the NOx sensor, its electronics module and the wiring: read the fault codes and see whether the sensor responds. On a diesel with an SCR efficiency fault, check the AdBlue injector and fluid quality as well.
• Whether the sensor can be removed afterwards depends on the ECU: it also measures oxygen, and on some systems the ECU uses that signal. Mention it in the order and we will advise.
• After flashing, clear faults in all modules and check after a drive with a fully warm engine that the NOx sensor faults have not come back.

How to order the file

1. Read the original file via OBD or on the bench with the tool you use. We need an unmodified stock file.
2. Upload it to the portal and select NOx OFF. You can add other options, such as Stage 1, at the same time.
3. Get the finished file with recalculated checksums, on average within 30–60 minutes.
4. Flash the file and clear the faults.

The option is available mainly for petrol ECUs including Bosch ME, MED and MEVD and Continental Simos, MSS and MSD, as well as some diesel ECUs, and the full list by make is below. If your ECU is not there, send the file with a request: we will check the software and tell you whether there is a solution.

Disabling NOx monitoring increases nitrogen oxide emissions and changes the vehicle's emissions class. In the EU, the UK, the US and a number of other countries this is prohibited on public roads and may cause an inspection failure. The service is intended for tuners and is used where the law allows it: motorsport, off-road, export, vehicles not used on public roads. Responsibility for using the file lies with the vehicle owner.