P3838moderate severityPowertrain
NOx Sensor Downstream of SCR Rationality Error
on 2010 Audi A3 2.0L TDI I4 · FWD
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.
Other Powertrain codes on this vehicle
P0001
Fuel Volume Regulator Control Circuit/Open
P0002
Fuel Volume Regulator Control Circuit Range/Performance
P0003
Fuel Volume Regulator Control Circuit Low
P0004
Fuel Volume Regulator Control Circuit High
P0010
Intake Camshaft Position Actuator Circuit/Open Bank 1
P0011
Intake Camshaft Position Timing Over-Advanced Bank 1
P0012
Intake Camshaft Position Timing Over-Retarded Bank 1
P0013
Exhaust Camshaft Position Actuator Circuit/Open Bank 1
P0014
Exhaust Camshaft Position Timing Over-Advanced Bank 1
P0015
Exhaust Camshaft Position Timing Over-Retarded Bank 1