P3836moderate severityPowertrain
DEF Tank Heater Circuit Malfunction
on 2010 Audi A3 2.0L TDI I4 · FWD
What this code means
The DEF tank and line heating system has a circuit fault. The heater prevents DEF from freezing (DEF freezes at 12F/-11C).
Symptoms
- DEF frozen in cold weather
- No DEF dosing in winter
- SCR system inoperative when cold
- Derate warning in cold conditions
- DEF warning light in winter
Common causes
- Heater element failure
- Heater relay fault
- Wiring damage from road debris
- Fuse blown
- Control module heater driver failure
Diagnostic steps
- Test DEF heater element resistance
- Check heater relay operation
- Inspect wiring to DEF tank heater
- Test coolant-based heater flow (if equipped)
- Verify heater activation below threshold temperature
Repair notes
Replace DEF tank heater element or relay. Inspect wiring for road damage. Critical for cold-climate operation.
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