P20B7

Reductant Heater A Control Circuit Low

P20B7 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater A Control Circuit Low. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P20B7
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P20B7 means

DTC P20B7 is set when the PCM or SCR module detects a lower-than-expected voltage or current on the control circuit for reductant heater A. DEF (diesel exhaust fluid) freezes at approximately -11 degrees Celsius, so the SCR system includes electric heating elements to thaw the fluid before dosing begins in cold conditions. Heater A refers to the primary or first heater circuit in systems that use multiple heating zones.

A low circuit condition typically indicates a short to ground in the heater control wiring, an internally shorted heater element drawing excessive current, or a failure of the relay or solid-state driver controlling the heater. The fault can also be set if the power supply to the heater control circuit drops below the module's expected operating range.

Failure of the reductant heater in cold climates can prevent DEF dosing entirely until ambient temperatures rise above freezing, resulting in elevated NOx emissions and potential NOx catalyst efficiency faults. The PCM will log P20B7 and may alert the driver via a warning indicator.

Common causes

Most-frequently reported root causes when P20B7 is logged.

  • 1
    Short to ground in the reductant heater A control or supply wiring.
  • 2
    Failed heater element with an internal short creating excessive current draw.
  • 3
    Defective relay or solid-state switch controlling heater A power.
  • 4
    Corroded or damaged heater connector causing a partial short.
  • 5
    Blown fuse in the heater supply circuit caused by an overload condition.
  • 6
    PCM or SCR module output driver failure for the heater A circuit.

Symptoms drivers notice

MIL or SCR warning lamp illuminated.
DEF system may be inoperative in cold weather due to frozen fluid.
Possible NOx catalyst efficiency or dosing fault codes stored alongside P20B7.
No visible drivability symptoms in warm ambient conditions.
Possible blown fuse in the reductant heater supply circuit.

How to diagnose P20B7

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and note accompanying reductant or NOx system codes.
  2. 2
    Check the fuse and relay in the reductant heater A supply circuit.
  3. 3
    Inspect the heater A wiring harness and connector at the DEF tank module for shorts or damage.
  4. 4
    Measure resistance of the heater A element and compare to specification to check for internal short.
  5. 5
    Back-probe the PCM or SCR module heater A output and verify expected control signal with ignition on.
  6. 6
    Repair wiring faults or replace the heater element or DEF module as indicated by test results.
  7. 7
    Clear DTCs, operate in cold conditions or simulate with a scan tool command if available, and confirm the heater activates normally.

Related powertrain codes

Frequently asked questions

At what temperature does the reductant heater activate?

The heater typically activates at cold start when ambient or DEF temperature is below approximately -5 to 0 degrees Celsius, varying by manufacturer calibration.

Can P20B7 cause the vehicle to enter inducement mode?

P20B7 itself is a heater circuit fault; inducement is triggered by confirmed low DEF quantity or NOx system failure. However, a non-functional heater can lead to DEF dosing failure in cold weather, which may eventually trigger inducement.

Is heater A the DEF tank heater or the DEF line heater?

Heater A typically refers to the primary heating element in the DEF tank. Some systems also have line and nozzle heaters designated as separate circuits, which would be covered by different suffixes or codes.

How do I tell if the heater element itself is faulty versus the wiring?

Disconnect the heater connector and measure resistance across the heater terminals. A reading near zero ohms (short) or infinite (open) with the connector unplugged from the harness confirms an element fault rather than a wiring fault.

Disabling P20B7 in software

RaceTune can permanently disable P20B7 — 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 P20B7 disable in our catalogue

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