What it means
The downstream NOx sensor on bank 1 (post-SCR) responds too slowly when NOx levels fall after a reductant injection event. The control unit measures the time between commanded reductant dose and the resulting NOx drop and finds it exceeds the diagnostic threshold, indicating an aged sensor element.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 1 sensor 2 is the downstream NOx sensor after the SCR catalyst. The control module watches for NOx to fall after reductant is injected and sets this code when the downstream sensor responds too slowly. The delay can come from the sensor, but poor SCR conversion or incorrect reductant dosing can also keep downstream NOx from falling as expected.
Common causes
Most likely first.
- NOx sensor element aged or thermally damaged
- SCR efficiency reduced (catalyst aged or contaminated)
- Reductant injector dosing drift
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 live upstream and downstream NOx readings around reductant dosing events to confirm the downstream response is slow.
- Identify bank 1 sensor 2 and inspect its connector, harness, and exhaust section for damage, corrosion, heat exposure, or leaks.
- Check for related reductant dosing, DEF quality, or SCR efficiency codes before condemning the sensor.
- Compare commanded reductant dosing with downstream NOx changes to determine whether dosing or catalyst performance may be contributing.
- Test the downstream sensor response and reductant injector operation according to the manufacturer's diagnostic procedure.
Common fixes
- Repair damaged wiring or poor connections at bank 1 sensor 2.
- Correct a reductant injector dosing fault if testing confirms it.
- Repair the exhaust or address SCR catalyst condition if diagnostics identify an efficiency problem.
- Replace bank 1 sensor 2 if it fails the response test and other causes are ruled out.
Can I keep driving?
Gentle driving may be possible if the vehicle runs normally, but continued driving can leave NOx emissions high. Treat AdBlue/DEF warnings and any restart countdown as urgent because 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.