P20B7Reductant Metering Unit Heater Control Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- Warning lamp at the start of a cold-weather heating phase
- DEF dosing delayed in winter
- Likely causes
- LikelyHeater element resistance too high (aged or partly damaged)
- PossibleContact resistance at the heater terminals
- PossibleHeater wire partly broken (few strands left)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €60 – €1,800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.