Powertrain, generic (extended)

P20B8 Reductant Metering Unit Heater Control Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P20B8 marks the upper limit of the metering unit heater circuit. The 2014 Touareg CNRB table from Volkswagen sets it when the conductance measured at heating start exceeds 0.74 siemens, meaning the element is below about 1.35 ohms, with additional limits at 0.96 and 0.52 siemens. Such high conductance suggests shorted heater windings, wires touching each other or fluid bridging the terminals, and it implies excess current that strains the driver stage and wiring. The control unit will normally switch the heater off for protection, so in winter the outcome resembles an open heater (P20B5). Makers that judge their outputs by voltage may use the same code for a short to battery instead.

Likely causes

Heater element internally shorted (too low resistance)
Heater wires chafed and touching each other
DEF or water bridging the heater terminals

Symptoms

  • Warning lamp as soon as heating is requested
  • Heater switched off, DEF stays frozen longer
  • Possibly a blown heater fuse

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–1,800
Time0.5–3 h

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

Parts involved

reductant metering unit heater

Related codes

P20B5P20B6P20B7P20BC

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