P20BC

Reductant Heater A Control Circuit High

P20BC is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater A Control Circuit High. 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
P20BC
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on)
Need P20BC disabled?
RaceTune permanently disables any OBD-II trouble code on supported ECUs — for motorsport, off-road, and export use.
→

What P20BC means

DTC P20BC — Reductant Heater A Control Circuit High — is a SAE generic powertrain code set when the Engine Control Module (ECM) detects a voltage level on the reductant (DEF/AdBlue) heater A control circuit that is higher than the expected operating range. The reductant heater warms the DEF fluid and supply lines in sub-zero temperatures to prevent freezing and maintain proper SCR system operation. A "circuit high" condition typically indicates a short to voltage in the control wiring, a failed heater element presenting abnormally high resistance on the return path, or an open in the low-side ground circuit forcing the sensed voltage above threshold.

P20BC belongs to a sibling family: P20B9 (open circuit), P20BA (range/performance), and P20BB (circuit low). Because the code is circuit-level rather than temperature-performance, the SCR system may be partially or fully inhibited by the ECM as a protective measure, particularly in cold ambient conditions. The fault is most commonly traced to harness damage near the DEF tank heater, connector corrosion at the heater connector or DEF module, a faulty reductant heater element, or internal ECM driver failure.

Vehicles equipped with SCR systems are subject to emissions regulations that may trigger a torque de-rate or increased AdBlue consumption warnings when this fault is active. Clearing the code without repairing the root cause will result in an immediate or short-drive reset. Diagnosis should begin with a thorough inspection of the reductant heater harness before component replacement.

Common causes

Most-frequently reported root causes when P20BC is logged.

  • 1
    Short to voltage in the reductant heater A control circuit wiring
  • 2
    Open circuit on the low-side (ground) return path of the heater circuit
  • 3
    Corroded or water-ingressed connector at the DEF tank heater assembly
  • 4
    Failed reductant heater element (out-of-specification resistance)
  • 5
    Damaged wiring harness near the DEF tank (chafing, heat damage, or rodent damage)
  • 6
    Faulty DEF supply module or integrated heater driver
  • 7
    ECM internal driver failure on the heater A control output

Symptoms drivers notice

MIL (Check Engine / Emissions warning light) illuminated
AdBlue/DEF system warning message on the instrument cluster
Reductant heater not operating in cold weather, leading to DEF freezing
Possible SCR system inhibition with a reduction in engine torque output
DEF level sensor or temperature readings may appear erratic if the heater module is integrated
No noticeable drivability issue under warm ambient conditions

How to diagnose P20BC

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and confirm P20BC is present; record all freeze-frame data including ambient temperature and DEF fluid temperature at time of fault
  2. 2
    Inspect the reductant heater A wiring harness and connector for visible damage, chafing, corrosion, or moisture intrusion — pay particular attention to the routing near the DEF tank
  3. 3
    With the circuit unplugged, measure heater element resistance across the heater connector terminals and compare to manufacturer specification (typically 1–10 Ω depending on heater wattage); replace heater if out of spec
  4. 4
    With KOEO (key on, engine off), backprobe the control circuit at the ECM connector and check for unexpected voltage on the low-side driver wire (should be near 0 V when commanded off)
  5. 5
    Check for a short to voltage between the heater control wire and any B+ supply wire in the same harness section using a DMM
  6. 6
    Repair any wiring faults found and retest; if the circuit checks within spec end-to-end, suspect the DEF supply module or ECM driver and consult OEM wiring diagrams before replacing

Vehicles where we've handled P20BC

Platforms in our catalogue with confirmed P20BC coverage.

AUDI A4 20D
VW TRANSPORTER
AUDI A6
2015
AUDI A6 30D
2015
AUDI A7
AUDI A7 30D

Related powertrain codes

Frequently asked questions

Can I drive with P20BC active?

In warm weather you can typically drive short distances, as the DEF heater is only needed in sub-freezing conditions. However, the MIL is on and the SCR system may be partially inhibited, so emissions compliance is affected. In cold climates, an inactive heater will allow DEF to freeze, which will eventually trigger additional faults and a torque de-rate. Repair promptly.

Is P20BC the same as P20BB?

No. P20BB indicates a circuit low condition (voltage below expected range, typically a short to ground), while P20BC indicates a circuit high condition (voltage above expected range, typically a short to voltage or an open ground return). Both point to wiring or heater element faults but require different circuit checks.

Will P20BC clear itself after repair?

The code will not clear itself automatically. After repairing the fault, use a scan tool to clear the DTC, then perform a drive cycle that includes cold-start conditions (if ambient temperature permits) so the ECM can re-evaluate the heater circuit and confirm the repair.

Could a low DEF level cause P20BC?

No. P20BC is a circuit electrical fault, not a fluid level fault. Low DEF level sets its own separate warning codes. If both are present simultaneously, address the circuit fault first, as it may be causing erroneous DEF module readings.

Disabling P20BC in software

RaceTune can permanently disable P20BC — 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 P20BC disable in our catalogue

Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.

  • Bosch EDC17C74 verified 2 software versions
  • Bosch EDC17CP44 verified 1 software version

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

Got P20BC in your scan?

Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.

Upload your file