P20C3

Reductant Heater C Control Circuit Low

P20C3 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater C 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
P20C3
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on)
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What P20C3 means

P20C3 is a general circuit fault code for the reductant injection air purge control system. Unlike the more specific low (P20C6) or high variants, P20C3 indicates an unspecified electrical fault detected in the air purge control circuit by the ECM, which could include open circuits, short circuits, or out-of-range signals on the control line.

The air purge solenoid is energized during the SCR system post-shutdown routine to blow residual urea solution from the dosing injector tip and supply lines back toward the tank, preventing crystallization that would block the injector. The ECM monitors the circuit by checking the feedback from the driver circuit against the commanded state.

Diagnosis of P20C3 follows the same general approach as other circuit faults: start with a visual inspection of the wiring harness, connectors, and solenoid body before performing resistance and voltage measurements. Because P20C3 is a generic circuit code rather than a direction-specific one, both high-resistance and low-resistance faults should be evaluated during testing.

Common causes

Most-frequently reported root causes when P20C3 is logged.

  • 1
    Open circuit in the wiring between ECM and the air purge solenoid.
  • 2
    Short circuit to ground or to voltage in the control wire.
  • 3
    Corroded or loose connector at the air purge solenoid or ECM harness.
  • 4
    Failed air purge solenoid coil (open or shorted winding).
  • 5
    Damaged wiring harness in the reductant module wiring routing.
  • 6
    Internal ECM driver fault affecting the air purge control output.
  • 7
    Water or fluid contamination inside the solenoid connector.

Symptoms drivers notice

MIL illuminated.
Reductant system warning message on dashboard.
Air purge cycle does not execute after engine shutdown.
Urea crystallization and injector clogging over extended periods of failed purging.
Possible related SCR dosing efficiency codes if injector becomes partially blocked.

How to diagnose P20C3

A typical diagnostic flow when this code is present.

  1. 1
    Scan for all DTCs and capture freeze frame data to establish the fault context.
  2. 2
    Inspect the air purge solenoid wiring harness and connectors visually for damage, corrosion, or moisture.
  3. 3
    Unplug the solenoid connector and measure coil resistance; compare the value to the manufacturer specification.
  4. 4
    Check for a short to ground or short to voltage on the control wire with the connector disconnected.
  5. 5
    Verify proper voltage supply and chassis ground at the solenoid connector harness side.
  6. 6
    If wiring and solenoid test pass, check ECM output voltage during commanded activation of the purge system.
  7. 7
    Repair identified faults, clear codes, and verify the air purge system operates correctly through a post-shutdown cycle.

Vehicles where we've handled P20C3

Platforms in our catalogue with confirmed P20C3 coverage.

AUDI A4 20D

Related powertrain codes

Frequently asked questions

What is the difference between P20C3 and P20C6?

P20C3 is a generic air purge circuit fault covering any electrical anomaly. P20C6 specifies a low voltage or short-to-ground condition. P20C3 may accompany either direction of fault depending on manufacturer implementation.

How urgent is P20C3?

The immediate drivability impact is low, but if the purge system remains inoperative, the dosing injector may clog with urea crystals over time, leading to SCR efficiency faults and potential derates.

Can I clear the code and keep driving?

Clearing the code without repair only delays further faults. Diagnose and repair the circuit fault to protect the dosing injector from urea buildup.

Does P20C3 require replacing the entire reductant module?

Not necessarily. The fault is circuit-based. Often a corroded connector or damaged wire is the cause. Only condemn the solenoid or module after wiring has been verified good.

Disabling P20C3 in software

RaceTune can permanently disable P20C3 — 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 P20C3 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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