NOx sensors sit in the hot exhaust stream and degrade over time. On many diesels — especially PSA, Mercedes and Vauxhall — they are the first component to fail and the primary trigger for AdBlue countdowns.
Symptoms of NOx sensor failure
AdBlue warning or countdown appearing suddenly.
Engine management light (EML) illuminated.
Reduced power / limp mode.
Fault codes such as P229F, P220A, P204F and P207F.
Replacement vs removal
A genuine sensor is £250–£500; many vehicles need two. Labour and coding often adds another £150+.
Even after replacement, sensors regularly fail again within 12–24 months — especially on short-journey vehicles.
Permanent AdBlue removal eliminates the sensor from the ECU's monitoring strategy, so future failures no longer trigger warnings.
Frequently asked questions
Which NOx sensor fails most often?＋
The downstream (post-catalyst) sensor is the most common failure, though the upstream sensor also fails on higher-mileage vehicles.
Can I drive with a failed NOx sensor?＋
Short-term yes, but the AdBlue countdown will start almost immediately and end in a no-start condition.