P20B8Reductant Metering Unit Heater Control Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- Warning lamp as soon as heating is requested
- Heater switched off, DEF stays frozen longer
- Possibly a blown heater fuse
- Likely causes
- LikelyHeater element internally shorted (too low resistance)
- PossibleHeater wires chafed and touching each other
- RareDEF or water bridging the heater terminals
- 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.