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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.

  • Check engine light: Yes
  • Emissions related: Yes
  • Repair difficulty: hard

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.

  • Short to B+Common
  • Connector pins corroded - high contact resistanceLess likely
  • Supply wire pinched against B+ sourceLess likely

Parts involved

  • Reductant Metering Unit Heater

P20B8 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.