Powertrain, generic (extended)

P20B6 Reductant Metering Unit Heater Control Circuit Performance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

P20B6 is the functional check of the metering unit heater circuit: wiring and driver pass the electrical tests, yet the heating falls short of what the control unit expects, for instance the module temperature barely rises during a commanded heating phase or the current drifts away from its target. Such a monitor needs the heater to be running, so it can only reach a verdict in cold conditions and across a drive cycle. Volkswagen's Touareg CNRB chart lists P20B5, P20B7 and P20B8 for this heater but no P20B6, and no other maker publishes the code with limits. Suspects include an aged element with changed resistance, poor thermal contact inside the module or a temperature sensor feeding a false value.

Likely causes

Aged heater element with reduced output
Metering unit temperature sensor reporting a wrong value
Low supply voltage to the heater (weak battery, poor ground)

Symptoms

  • Warning lamp after cold starts only
  • Long delay before DEF dosing begins in frost

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–1,800
Time1–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant metering unit heater

Related codes

P20B5P20B7P20B8P20BAP209F

References: law.cornell.edu, static.nhtsa.gov, picoauto.com.