OBD-II code · Powertrain (generic, extended)
P20B4 Reductant Heater Coolant Control Valve Circuit High
Reductant coolant control valve drive line stuck high — short to B+. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
Is it safe to drive with P20B4?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyShort to B+
- Less likelyConnector pins corroded - high contact resistance
- Less likelySupply wire pinched against B+ source
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)
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.7–2.5 h
- Emissions test
- Likely to fail
- Parts involved
- Reductant Coolant Valve
Related codes
- P20B1Reductant Heater Coolant Control Valve Circuit/Open
- P20B2Reductant Heater Coolant Control Valve Performance/Stuck Open
- P20B3Reductant Heater Coolant Control Valve Circuit Low
- P20B5Reductant Metering Unit Heater Control Circuit/Open
- P20B6Reductant Metering Unit Heater Control Circuit Performance
- P20B7Reductant Metering Unit Heater Control Circuit Low
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.