P20B6: Reductant Metering Unit Heater Control Circuit Performance

P20B6 is the functional check of the metering unit heater circuit: wiring and driver pass the electrical tests, yet the heating falls short of what the control unit expects, for instance the module temperature barely rises during a commanded heating phase or the current drifts away from its target. Such a monitor needs the heater to be running, so it can only reach a verdict in cold conditions and across a drive cycle. Volkswagen's Touareg CNRB chart lists P20B5, P20B7 and P20B8 for this heater but no P20B6, and no other maker publishes the code with limits. Suspects include an aged element with changed resistance, poor thermal contact inside the module or a temperature sensor feeding a false value.

Symptoms

• Warning lamp after cold starts only
• Long delay before DEF dosing begins in frost

Most likely causes

• Aged heater element with reduced output
• Metering unit temperature sensor reporting a wrong value
• Low supply voltage to the heater (weak battery, poor ground)

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. Compare the reading with other engine data while varying the engine speed.
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 P20B6 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 0: Subsystem depends on the code range
• B6: Specific fault in that system

Frequently asked questions

Can I keep driving with code P20B6?

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

What causes code P20B6?

The most common causes are: aged heater element with reduced output; metering unit temperature sensor reporting a wrong value; low supply voltage to the heater (weak battery, poor ground).

Related codes

• P20B5Reductant Metering Unit Heater Control Circuit/Open
• P20B7Reductant Metering Unit Heater Control Circuit Low
• P20B8Reductant Metering Unit Heater Control Circuit High
• P20BAReductant Heater A Control Circuit Performance
• P209FReductant Tank Heater Control Circuit Performance

Sources

• law.cornell.edu: 13 CCR 1968.2
• static.nhtsa.gov: MC 10074008 2280
• picoauto.com: bosch denoxtronic 2 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.