P20B8: Reductant Metering Unit Heater Control Circuit High

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.

Symptoms

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

Most likely causes

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

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the AdBlue/DEF level and quality, and do not add additives or water
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Look for white crystals around the injector and the tank
4. Check connectors and heaters with the scanner
5. Run the dosing test available on your diagnostic tool
6. Replace parts only when the test points to that component

Good to know

• Repair difficulty: hard
• It can trigger limp mode.
• It affects emissions.

What each character of P20B8 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 0: Subsystem depends on the code range
• B8: Specific fault in that system

Frequently asked questions

Can I keep driving with code P20B8?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P20B8?

The most common causes are: heater element internally shorted (too low resistance); heater wires chafed and touching each other; dEF or water bridging the heater terminals.

Related codes

• P20B5Reductant Metering Unit Heater Control Circuit/Open
• P20B6Reductant Metering Unit Heater Control Circuit Performance
• P20B7Reductant Metering Unit Heater Control Circuit Low
• P20BCReductant Heater A Control Circuit High

Sources

• static.nhtsa.gov: MC 10074008 2280
• law.cornell.edu: 13 CCR 1968.2

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.