P229F: NOx Sensor Circuit Range/Performance (Bank 1 Sensor 2)
Severity: medium
What it means: The downstream NOx sensor (the one after the SCR catalyst that uses DEF/AdBlue to clean diesel exhaust) is sending a reading that is electrically valid but doesn't make sense. It can mean the sensor is bad, its wiring is damaged, or the DEF system really isn't cleaning the exhaust well.
Safe to drive? Usually drivable for a short time, but on many diesels an emissions fault starts a countdown that can limit speed or prevent restarting. Get it diagnosed soon.
Common symptoms
- Check-engine light with P229F
- DEF/AdBlue warning or a 'starts remaining' countdown
- Reduced power on some vehicles
Likely causes, ranked
- Failed or contaminated NOx sensorShop
- Damaged sensor wiring or connectorShop
- DEF dosing or SCR catalyst problemShop
- Exhaust leak near the sensorShop
Vehicle-specific notes
- :
DIY or shop?
Diagnosis needs live sensor data and often dealer-level software to code a new sensor and reset the DEF system.
Before you replace parts
- Make sure the DEF tank has fresh, sealed DEF.
- Compare upstream and downstream NOx readings with a capable scan tool.
- Check the wiring and connector at the sensor.
- After replacing a sensor, have it coded and the DEF system reset as the maker requires.
Vehicle notes
- Mercedes-Benz Sprinter diesel (2019–2023): NOx sensor faults are a common reason for the AdBlue 'starts remaining' countdown. See the related Sprinter answer.
Sources
Safety / mechanic note
An emissions countdown that reaches zero can leave the vehicle unable to restart. Verify with a mechanic. A code points to a circuit or system, not always a specific part.