OBD-II code · Powertrain (generic, extended)
P20B8 Reductant Metering Unit Heater Control Circuit High
Metering unit heater control line stuck high — short to B+. The sensor heater warms the element to operating temperature (~600 °C); without it the lambda sensor stays inactive in closed loop and the ECM cannot trim mixture.
Is it safe to drive with P20B8?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
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
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1–4 h
- Emissions test
- Likely to fail
- Parts involved
- Reductant Metering Unit Heater
Related codes
- P20B5Reductant Metering Unit Heater Control Circuit/Open
- P20B6Reductant Metering Unit Heater Control Circuit Performance
- P20B7Reductant Metering Unit Heater Control Circuit Low
- P20B9Reductant Heater A Control Circuit/Open
- P20BAReductant Heater A Control Performance
- P20BBReductant Heater A Control Circuit Low
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.