P20BF

Reductant Heater B Control Circuit Low

P20BF is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater B Control Circuit Low. It is logged by the engine control unit when the scr/adblue monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P20BF
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on)
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What P20BF means

DTC P20BF is set when the PCM or SCR module detects a lower-than-expected voltage or current on the reductant heater B control circuit. Heater B is the second heating circuit in SCR systems that use multiple discrete heating zones, such as a separate line heater or a secondary tank heating element, in addition to the primary heater A circuit covered by P20B7.

A low circuit condition points to a short to ground somewhere in the heater B wiring, a shorted heater element, a failed switching device, or a power supply fault specific to the B circuit. On vehicles where heater B is the DEF supply line heater, the fault can result in the DEF line remaining frozen in severe cold even if the tank heater is operating correctly, preventing proper fluid delivery to the dosing injector.

Technicians should cross-reference P20BF with any active NOx catalyst efficiency codes to determine whether the heater fault is already impacting DEF delivery. Diagnosis follows the same methodology as heater A faults but targets the heater B wiring, fuse, relay, and element.

Common causes

Most-frequently reported root causes when P20BF is logged.

  • 1
    Short to ground in the heater B control or supply wiring harness.
  • 2
    Internally shorted reductant heater B element.
  • 3
    Failed relay or solid-state driver for the heater B circuit.
  • 4
    Blown fuse specific to the heater B supply branch.
  • 5
    Corroded connector terminals at the heater B junction causing a current leak.
  • 6
    PCM or SCR controller output driver failure on the heater B channel.

Symptoms drivers notice

MIL or SCR/DEF system warning lamp illuminated.
DEF line or secondary tank zone may remain frozen in cold weather.
Possible accompanying NOx efficiency or DEF dosing fault codes.
Blown fuse may be found in the heater B power circuit.
No noticeable drivability impact at ambient temperatures above freezing.

How to diagnose P20BF

A typical diagnostic flow when this code is present.

  1. 1
    Scan all modules and record all DEF and NOx related DTCs to establish the full fault context.
  2. 2
    Locate and inspect the fuse and relay for the heater B supply circuit.
  3. 3
    Inspect the heater B wiring harness along its routing for chafing, pinching, or moisture intrusion.
  4. 4
    Disconnect the heater B connector and measure element resistance to check for internal short.
  5. 5
    Measure voltage at the heater B connector supply terminal with ignition on to verify power delivery.
  6. 6
    Test the heater B control signal at the SCR module output with the heater disconnected.
  7. 7
    Repair identified wiring faults or replace the heater element, relay, or module as needed, then verify with a cold-start functional test.

Related powertrain codes

Frequently asked questions

What is the difference between P20B7 and P20BF?

P20B7 is the low circuit fault for heater A (typically the main tank heater) and P20BF is the same fault type for heater B (typically a line or secondary zone heater). Both indicate a short-to-ground or excessive current condition on their respective circuits.

Can I ignore P20BF in summer months?

The code should still be repaired even in warm weather. A shorted heater element or wiring can cause repeated fuse failures, and the fault must be cleared for the SCR system monitors to run to completion.

Does P20BF affect emissions in warm weather?

In ambient temperatures above freezing, the heater is not needed for DEF flow, so NOx emissions are typically unaffected. In cold weather, a failed heater B can prevent DEF dosing, raising NOx.

Is the heater B element a dealer-only part?

Availability varies by manufacturer. On some vehicles the DEF tank module including all heaters is sold as an assembly; on others individual heating circuits are serviceable. Consult the parts catalog for the specific vehicle.

Disabling P20BF in software

RaceTune can permanently disable P20BF — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECUs with a P20BF disable in our catalogue

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