NOx Sensor Downstream of SCR Rationality Error
2009-2013 Audi A3 2.0L TDI
Description
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.
Related Codes
Other Powertrain Codes
- P0001Fuel Volume Regulator Control Circuit/Open
- P0002Fuel Volume Regulator Control Circuit Range/Performance
- P0003Fuel Volume Regulator Control Circuit Low
- P0004Fuel Volume Regulator Control Circuit High
- P0010Intake Camshaft Position Actuator Circuit/Open Bank 1
- P0011Intake Camshaft Position Timing Over-Advanced Bank 1
- P0012Intake Camshaft Position Timing Over-Retarded Bank 1
- P0013Exhaust Camshaft Position Actuator Circuit/Open Bank 1
- P0014Exhaust Camshaft Position Timing Over-Advanced Bank 1
- P0015Exhaust Camshaft Position Timing Over-Retarded Bank 1
P3838 on Other Vehicles
Common Questions
What does code P3838 mean on a 2009-2013 Audi A3?
P3838 stands for "NOx Sensor Downstream of SCR Rationality Error". The downstream NOx sensor reading does not correlate with expected values based on DEF dosing rate and upstream NOx levels.
What are the symptoms of P3838 on a Audi A3?
Common symptoms of P3838 include: Check Engine Light, Incorrect SCR efficiency calculation, False derate warnings possible, DEF dosing may be incorrect, Emissions test may show inconsistent results.
What causes P3838 on a 2009-2013 Audi A3?
Common causes of P3838 include: NOx sensor drift or failure, Sensor cross-sensitivity to ammonia, Sensor wiring intermittent, Exhaust leak near sensor, Sensor poisoned by contaminants.