What it means
NOx sensor on bank 1 sensor 1 responds too slowly when NOx concentration falls (typically observed when SCR dosing increases). The sensor has aged or its protection cap is clogged.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 1 sensor 1 is the upstream NOx sensor before the SCR catalyst. The control module watches how quickly its reported NOx level falls when exhaust NOx is expected to decrease, often after reductant dosing increases. This code sets when that high-to-low response is too slow, which can impair emissions control.
Common causes
Most likely first.
- Sensor aged
- Sensor element aged beyond calibration window
- Sensor element contaminated by oil, soot or coolant
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Save freeze-frame data and review live NOx readings and reductant commands to establish when the slow falling response was detected.
- Locate bank 1 sensor 1 using service information and inspect its connector and harness for corrosion, loose pins, or exhaust heat damage.
- Check for exhaust leaks and visible contamination around the upstream sensor that could affect its readings.
- Compare the sensor's response with reductant dosing changes and the downstream NOx sensor using a scan tool.
- If the circuit and exhaust are intact, evaluate bank 1 sensor 1 response using the manufacturer's diagnostic procedure.
Common fixes
- Repair corroded terminals or damaged wiring at bank 1 sensor 1.
- Repair an exhaust leak affecting the upstream NOx measurement, if present.
- Replace bank 1 sensor 1 if it fails the response test.
- Correct the source of oil, soot, or coolant contamination if identified.
Can I keep driving?
Gentle driving is generally possible if the vehicle behaves normally, but the emissions system may not control NOx correctly. Respond promptly to AdBlue/DEF alerts or a restart countdown, since some vehicles may eventually prevent restarting.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.