NOx sensor fault: symptoms, codes and what to do
A NOx sensor fault means one of the sensors measuring nitrogen oxides in your diesel exhaust has failed or is sending readings the ECU won't trust. Those readings decide how much AdBlue is injected and whether the system is working, so a bad sensor usually brings on an AdBlue or emissions warning. On most Euro 6 vehicles, a restart countdown follows.
What NOx sensors do
Most diesels with AdBlue have two NOx sensors. The upstream sensor sits before the SCR catalyst and measures what the engine produces, and the downstream sensor sits after it and measures what's left.
The ECU compares the two to check AdBlue is doing its job. Each sensor has a small control module on its wiring, and a fault in either the probe or the module sets a code.
NOx sensor symptoms
- AdBlue or engine management light on
- An emission system fault or "check AdBlue system" message
- A mileage countdown to no-restart on the dash
- Higher or lower AdBlue use than normal
- Reduced power on some makes
- Codes such as P2200, P20EE or P2BAD
Why NOx sensors fail
The sensors live in a hot exhaust and run their own internal heater, so age and repeated heat cycles take their toll. Soot from a blocked DPF or a sticking EGR valve can coat the upstream sensor, and road salt gets into the connectors and module.
Water in the exhaust on a cold start can also crack the sensor element. A fault in the wiring or the module accounts for plenty of NOx sensor failure codes, so the probe itself isn't always to blame.
Codes linked to NOx sensors
| Code | What it means |
|---|---|
| P2200 | NOx sensor circuit (bank 1) |
| P2201 | NOx sensor circuit range/performance (bank 1) |
| P2202 | NOx sensor circuit low (bank 1) |
| P229F | NOx sensor circuit range/performance (bank 1, sensor 2) |
| P20EE | SCR NOx catalyst efficiency below threshold (bank 1), so the catalyst isn't cutting NOx enough |
| P2BAD | NOx exceedance, root cause unknown |
Sensor 1 is the upstream sensor and sensor 2 the downstream one on most vehicles.
Is it really the sensor?
P20EE and P2BAD tell you the NOx level after the catalyst is too high. That can be a lazy sensor, but it's just as often poor AdBlue, a crystallised injector, a failing pump or an SCR catalyst that has worn out.
Before a sensor is ordered, it's worth checking live readings from both sensors, the AdBlue quality and a dosing test. Sensors aren't cheap, and fitting one that wasn't faulty won't clear the warning.
What to do with a NOx sensor code
Write down every code
A sensor circuit code and a P20EE together tell a different story to P20EE on its own.
Check the AdBlue
Low or old fluid can make a healthy sensor look faulty.
Look for exhaust soot problems
A blocked DPF or sticking EGR valve can foul the upstream sensor.
Have both sensors compared
Live readings show whether one sensor disagrees with the other or the whole system is under-dosing.
NOx sensor replacement and cost
What's involved
The sensor and its module are usually replaced as one part. Some makes need the new sensor coded or adapted with a diagnostic tool afterwards.
What affects the price
NOx sensor replacement cost varies with the make, whether a genuine or aftermarket part is used and how easy the sensor is to reach.
Who does it
SGR don't replace NOx sensors. A dealer or SCR specialist can, and our diagnosis tells them which sensor has failed.
Can I keep using the car with a faulty NOx sensor?
In the short term, yes. Most vehicles start a countdown once the fault is confirmed, and the engine won't restart when it reaches zero.
Can a NOx sensor be cleaned?
They're sealed units, so cleaning rarely brings one back. If soot caused the failure, the DPF or EGR problem behind it needs fixing too.
How do I know if it's the upstream or downstream sensor?
The code and the sensor number point to it, and live data confirms it. Sensor 1 is usually upstream of the catalyst and sensor 2 downstream.
Can a NOx sensor fault cause limp mode?
On some makes, yes. The ECU may cut power to limit emissions until the fault is cleared, alongside the AdBlue warning.
Will a new NOx sensor stop the AdBlue countdown?
Once the faulty part is replaced and the codes cleared, the countdown should stop. Some vehicles need a reset with a diagnostic tool.
Can the AdBlue system be deleted instead of replacing a NOx sensor?
Only on a vehicle that's used off-road or exported, because deletes are sold for off-road and export use only. Our guide to whether an AdBlue delete is legal sets out the rules.
Find out which sensor has failed
We'll read the codes and compare live data from both sensors at your home or workplace, so you only pay for the part that's actually faulty. If you'd rather not replace the sensor, our AdBlue delete is £330 + VAT.
Eccles, Manchester M30 0SF