OBD-II code · Powertrain (generic, extended)
P20B7 Reductant Metering Unit Heater Control Circuit Low
Metering unit heater control line stuck low — short to ground. 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 P20B7?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyShort to ground
- Less likelyConnector pins corroded - high contact resistance
- Less likelyHarness chafing through insulation
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
- P20B4Reductant Heater Coolant Control Valve Circuit High
- P20B6Reductant Metering Unit Heater Control Circuit Performance
- P20B8Reductant Metering Unit Heater Control Circuit High
- P20B9Reductant Heater A Control Circuit/Open
- P20BAReductant Heater A Control Performance
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.