P3838moderate severityPowertrain
NOx Sensor Downstream of SCR Rationality Error
on 2010 Audi Q7 3.0L TDI V6 · AWD
What this code means
The downstream NOx sensor reading does not correlate with expected values based on DEF dosing rate and upstream NOx levels.
Symptoms
- Check Engine Light
- Incorrect SCR efficiency calculation
- False derate warnings possible
- DEF dosing may be incorrect
- Emissions test may show inconsistent results
Common causes
- NOx sensor drift or failure
- Sensor cross-sensitivity to ammonia
- Sensor wiring intermittent
- Exhaust leak near sensor
- Sensor poisoned by contaminants
Diagnostic steps
- Compare upstream and downstream NOx sensor readings
- Check sensor readings at key operating points
- Test sensor with known gas concentration
- Inspect sensor wiring and connector
- Check for exhaust leaks near sensor
Repair notes
Replace downstream NOx sensor. Verify SCR efficiency calculation normalizes after replacement. Use OEM sensor only.
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