P20B5: Reductant Metering Unit Heater Control Circuit/Open

P20B5 concerns the electric heater inside the reductant metering unit, the element that keeps the pump module from freezing. The control unit sets it when this heater circuit is open or otherwise broken. Volkswagen documents the code for the 2014 Touareg 3.0 TDI (engine CNRB): the engine control unit measures the conductance of the element while heating and stores P20B5 at 0.10 siemens or less, roughly 10 ohms or more. A burnt-out element, a broken wire or a corroded terminal are the usual reasons. With the heater dead, DEF in the module stays frozen longer after a cold start and dosing begins late. Too little and too much conductance have separate codes, P20B7 and P20B8, and the tank heater uses its own range.

Medium severity. Can I keep driving?Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

Symptoms

Warning lamp mainly in cold weather
DEF dosing starts late after a cold start
Inducement countdown possible

Most likely causes

Heater element in the metering unit burnt out (usually module replacement)
Broken heater supply or ground wire
Corroded heater terminals at the module connector

What to check, in order

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

Check the AdBlue/DEF level and quality, and do not add additives or water
Check the continuity of the wire between the component and the control module.
Look for white crystals around the injector and the tank
Check connectors and heaters with the scanner
Run the dosing test available on your diagnostic tool
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 P20B5 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
0: Subsystem depends on the code range
B5: Specific fault in that system

Frequently asked questions

Can I keep driving with code P20B5?

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

What causes code P20B5?

The most common causes are: heater element in the metering unit burnt out (usually module replacement); broken heater supply or ground wire; corroded heater terminals at the module connector.

Related codes

Sources

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.