P20B6 — Reductant Metering Unit Heater Control Circuit Performance #

The code label may vary depending on the vehicle manufacturer.

• Reductant Metering Unit Heater Circuit Range/Performance (Ford, GM, FCA, Mercedes-Benz)
• DEF Dosing Unit Heater Control Circuit Performance (BMW, VW/Audi)
• AdBlue Metering Unit Heater Circuit Out of Range (Renault, Nissan, Peugeot/Citroën)

Summary of Code P20B6 #

The diagnostic trouble code P20B6 indicates an issue in the control circuit for the Reductant Metering Unit Heater. Simply put, this heater is a key component in the Selective Catalytic Reduction (SCR) system, responsible for keeping the DEF (Diesel Exhaust Fluid) from freezing in cold conditions. When the engine control module (ECM) or powertrain control module (PCM) detects a performance problem—whether the circuit is underperforming, overperforming, or behaving erratically—this code is triggered. In most vehicles, this code will also trigger the Check Engine Light (CEL) and may affect emissions compliance. While the code does not always prevent the vehicle from starting, it can compromise the effectiveness of the emissions system and, in some cases, can lead to reduced engine power or a no-start condition if left unaddressed.

Common Symptoms for Code P20B6 #

Drivers and technicians may notice several symptoms when code P20B6 is present. While some signs are subtle, others are more obvious, especially in colder climates where DEF freezing can occur. These symptoms include:

• Check Engine Light (CEL) illuminated
• Warning lights or messages related to the SCR/DEF system (e.g., “Service DEF System”)
• Reduced engine power or “limp” mode
• Difficulty starting in cold weather
• Increased emissions or failed emissions test

Probable Causes of Code P20B6 #

The most common causes for code P20B6 relate to faults within the heater circuit itself or the wiring/connectors that supply it. Here they are, in order of frequency:

1. Open or short circuit in the heater wiring (damaged wire, corrosion, loose connection)
2. Failed reductant metering unit heater (the element itself is inoperative or out of spec)
3. Faulty relay or fuse for the heater circuit
4. Defective control module (PCM/ECM issue, rare but possible)
5. Poor ground or high resistance in the circuit

Step-by-Step Diagnostic for Code P20B6 #

Diagnosing P20B6 requires a systematic approach using basic tools and a scan tool capable of reading live data and freeze frame information. Follow these steps:

• Verify the code and check for related DTCs (especially other DEF/SCR or heater codes).
• Visually inspect the wiring harness and connectors at the metering unit and along the heater circuit for damage, corrosion, or loose connections.
• Check the heater element resistance using a multimeter; compare to manufacturer specs (usually found in the repair manual or OEM data).
• Test for voltage and ground at the heater connector with the ignition on and/or engine running (according to OEM procedure).
• Inspect and test any related fuses and relays in the circuit.
• Use bi-directional scan tool controls to command the heater on/off, watching for response and circuit changes.
• If all else checks out, consider PCM/ECM driver issues as a last resort (rare).

Possible Repairs for Code P20B6 #

The repair will depend on the root cause identified during diagnosis. Here are the most common solutions, linked to the causes above:

• Repair or replace damaged wiring or connectors in the heater circuit
• Replace the reductant metering unit heater if found faulty
• Replace a blown fuse or faulty relay dedicated to the heater circuit
• Clean corroded contacts or repair poor grounds
• If verified, replace the PCM/ECM (only after all other options are exhausted and confirmed by OEM tests)

Can You Drive with Code P20B6? #

Limited: While the vehicle will often remain drivable with code P20B6, it is not recommended to ignore the warning. In cold weather, DEF may freeze, causing further SCR system faults and potentially triggering reduced power or no-start modes in modern diesel vehicles. Long-term driving with this code may also result in failed emissions tests and possible legal compliance issues. Address the problem as soon as possible to avoid inconvenience and costly repairs.

Vehicles Most Commonly Affected by Code P20B6 #

This code is most frequently reported on late-model diesel vehicles equipped with SCR systems, especially those using DEF/AdBlue. Based on field experience and technical bulletins, the following makes and models are most often affected:

• Ford Super Duty (F-250/F-350) 6.7L PowerStroke (2011+)
• Chevrolet Silverado/GMC Sierra Duramax (2011+)
• Ram 2500/3500 Cummins Diesel (2013+)
• Mercedes-Benz Sprinter Diesel (2010+)
• BMW X5/X3 Diesel models (2014+)
• VW/Audi TDI (2012+)
• Renault Master/Trafic Diesel (2015+)

Common Mistakes & Best Practices for Code P20B6 #

Some errors are easily avoided with a careful approach. Here’s how to sidestep common pitfalls and work efficiently:

• Assuming the metering unit heater is faulty without testing the wiring (often the real issue is a corroded connector or broken wire)
• Replacing the PCM/ECM prematurely (this is rarely the cause—always exhaust other diagnostic steps first)
• Ignoring the importance of proper ground and fuse checks
• Failing to clear codes and verify repair under real cold-start conditions
• Not consulting OEM service information for exact specs and test procedures

Express FAQ: Code P20B6 #

Here are quick answers to common questions about this code:

• Q: Can a faulty DEF heater cause engine performance problems?
  A: Yes, especially in cold weather. If the DEF freezes, the SCR system won’t function properly, which can trigger reduced power or limp mode on some vehicles.
• Q: Is this repair expensive?
  A: It varies. Simple wiring repairs or fuse replacements are low-cost, but replacing the metering unit heater or control module can be more expensive—consult a qualified technician for an accurate estimate.