P20C1

Reductant Heater C Control Circuit/Open

P20C1 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater C Control Circuit/Open. 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
P20C1
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on)
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What P20C1 means

P20C1 indicates that the ECM or aftertreatment control module has detected a lower-than-expected voltage or current on the reductant (DEF/AdBlue) injector control circuit for Bank 1. The reductant injector is the dosing valve that sprays DEF into the exhaust upstream of the SCR catalyst to enable the NOx reduction reaction.

A low circuit condition typically points to a short to ground in the injector wiring, an internally shorted injector solenoid winding, or a fault in the driver circuit within the control module. Because the injector must operate precisely to deliver the correct DEF quantity, a circuit fault will disable dosing and allow NOx emissions to rise above legal limits.

Diagnosis begins with an electrical inspection of the injector connector and harness for chafing or moisture ingress. Resistance measurement of the injector solenoid winding should be compared against the manufacturer specification. If wiring and connector checks are clean, substituting a known-good injector is a reliable confirmation step before suspecting the control module driver.

Common causes

Most-frequently reported root causes when P20C1 is logged.

  • 1
    Short to ground in the reductant injector signal or power wiring.
  • 2
    Internally shorted reductant injector solenoid coil.
  • 3
    Damaged or corroded injector connector causing a low-resistance path to ground.
  • 4
    Chafed wiring harness shorting against the exhaust or chassis.
  • 5
    Faulty aftertreatment or SCR control module injector driver circuit.
  • 6
    Moisture or contamination ingress into the injector connector.

Symptoms drivers notice

MIL or SCR warning lamp illuminated.
DEF dosing disabled, leading to increased NOx emissions.
Possible progressive engine derate if SCR system remains non-functional.
No perceptible driveability change in early stages.

How to diagnose P20C1

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and review freeze-frame data with a scan tool.
  2. 2
    Inspect the reductant injector harness and connector for shorts, chafing, or moisture.
  3. 3
    Measure resistance of the reductant injector solenoid coil and compare to specification.
  4. 4
    Check for continuity to ground on the injector signal wire with the harness disconnected from the module.
  5. 5
    Inspect the DEF supply line to the injector for blockage or freezing.
  6. 6
    Substitute a known-good reductant injector and retest if wiring checks pass.
  7. 7
    Inspect the aftertreatment control module connector and, if all other checks pass, consider module replacement.

Vehicles where we've handled P20C1

Platforms in our catalogue with confirmed P20C1 coverage.

AUDI A4 20D

Related powertrain codes

Frequently asked questions

Can a frozen DEF line cause P20C1?

A frozen line can prevent injector operation but typically sets a different code; P20C1 specifically indicates a circuit low (electrical) condition rather than a flow or pressure fault.

Is the reductant injector the same as a fuel injector?

No. The reductant injector doses aqueous urea solution into the exhaust stream and operates at much lower pressures than a fuel injector; they are not interchangeable.

Will P20C1 cause a failed emissions test?

Yes. An active MIL and disabled SCR dosing will result in excessive NOx and an automatic OBD-II emissions test failure.

How do I know if the injector is shorted vs the wiring?

Disconnect the injector and measure solenoid resistance; a value well below specification indicates an internally shorted injector, while an out-of-spec reading at the harness side (with injector disconnected) points to wiring.

Disabling P20C1 in software

RaceTune can permanently disable P20C1 — 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 P20C1 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 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.

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