P3831moderate severityPowertrain
DEF (Diesel Exhaust Fluid) Quality Sensor Malfunction
on 2010 Audi A3 2.0L TDI I4 · FWD
What this code means
The DEF quality sensor that measures urea concentration in the DEF tank is malfunctioning.
Symptoms
- DEF quality warning
- Potential derate countdown
- Check Engine Light
- Unable to verify DEF concentration
- Service DEF system message
Common causes
- DEF quality sensor failure
- Sensor contaminated by crystallized DEF
- Sensor wiring fault
- Sensor connector corrosion
- Diluted or incorrect DEF affecting sensor
Diagnostic steps
- Read DEF quality sensor PID
- Test sensor with known-good DEF
- Inspect sensor for crystallization
- Check sensor wiring and connector
- Compare sensor reading to refractometer test
Repair notes
Replace DEF quality sensor. Clean sensor mounting area of crystallized DEF. Use only API-certified DEF.
🔌 Don't have a code reader?
Any of these will read and clear this code — pick by how deep you want to go.
Bluetooth code reader (~$25)
Veepeak OBDCheck BLE →
Pairs with your phone — reads and clears codes, live data.
Serious DIY (~$100)
BlueDriver Pro →
Enhanced codes (ABS, airbag, TPMS) plus repair reports.
Shop-grade tablet (~$300+)
Autel MaxiCOM MK808S →
Bidirectional tests, service resets, all systems.
As an Amazon Associate, OLP earns from qualifying purchases — how we link. This never changes the specs we publish.
Other Powertrain codes on this vehicle
00532
Supply Voltage B+ Signal too Low
16394
Camshaft/Crankshaft Position Correlation Bank 1
16395
Camshaft Position Actuator Circuit/Open Bank 1
16486
Mass or Volume Air Flow Circuit Low Input
16487
Mass or Volume Air Flow Circuit High Input
16500
Engine Coolant Temperature Circuit Malfunction
16501
Engine Coolant Temperature Circuit Low Input
16502
Engine Coolant Temperature Circuit High Input
16514
O2 Sensor Circuit Bank 1 Sensor 1
16515
O2 Sensor Circuit Low Voltage Bank 1 Sensor 1