P20B7 Reductant Metering Unit Heater Control Circuit Low

P20B7 means the metering unit heater circuit reads low. Volkswagen gives this a concrete meaning for the 2014 Touareg CNRB: the control unit checks heater conductance at the start of heating and flags P20B7 below 0.33 siemens, so the element shows more than about 3 ohms; further stages at 0.18 and 0.04 siemens follow. In VW's logic, low therefore refers to conductance, not to voltage, and points to a degraded element, a high-resistance joint or a partly broken wire rather than a short to ground. Other makers may define the low case through the output-stage voltage instead, since their drivers differ. The module receives too little heat, and thawing after a frosty night takes longer.

Likely causes

Symptoms

• Warning lamp at the start of a cold-weather heating phase
• DEF dosing delayed in winter

Repair estimate

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

Parts involved

Related codes

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