What it means
The downstream NOx sensor on bank 1 (after the SCR catalyst) returns implausible NOx readings during operation. Either the sensor is degraded or upstream chemistry is abnormal.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
- The engine or gearbox may go into limp mode
How it works
Bank 1 sensor 2 is the downstream NOx sensor after the SCR catalyst, where it helps the control module assess how well the system is reducing nitrogen oxides. This code sets when its readings are implausible for operating conditions, which can point to an aged sensor or poor SCR performance rather than a simple electrical failure.
Common causes
Most likely first.
- NOx sensor aging
- Low SCR efficiency
- Sensor element aged beyond calibration window
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
- Review freeze-frame data and related SCR or DEF codes to establish engine load, exhaust temperature, and system conditions when the reading became implausible.
- Check scan-tool data for bank 1 sensor 2 NOx readings and compare them with upstream readings and the expected SCR response.
- Inspect the downstream sensor connector and harness for heat damage, corrosion, or chafing that could distort the signal.
- Check for exhaust leaks and review DEF quality or dosing faults that could reduce SCR conversion.
- If the wiring and SCR inputs check out, assess bank 1 sensor 2 against the manufacturer's diagnostic procedure for sensor aging or calibration drift.
Common fixes
- Repair damaged wiring or a poor connector at bank 1 sensor 2.
- Repair an exhaust leak affecting the downstream reading.
- Correct a confirmed DEF supply or SCR dosing fault.
- Replace the downstream NOx sensor if it is aged or fails the manufacturer's performance checks.
Can I keep driving?
Limit driving and arrange diagnosis soon; this fault can trigger reduced performance and an emissions-system start countdown. Follow any dashboard countdown or operating restriction, and avoid assuming that topping up DEF will correct an implausible sensor reading.
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.