P20B2
Reductant Heater Coolant Control Valve Performance/Stuck Open
Reductant coolant control valve is electrically OK but stuck open or does not modulate as commanded — DEF heating loop loses control authority.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
Common causes
Ordered by how often each turns out to be the cause.
- Valve stuck openCommon
- Carbon or debris buildup preventing free movementLess likely
- Component wear with age - mechanical playLess likely
Parts involved
- Reductant Coolant Valve
P20B2 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.