P20C4

Reductant Heater C Control Circuit High

P20C4 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater C 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
P20C4
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on)
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What P20C4 means

P20C4 is set when the ECM detects a high voltage condition in the reductant injection air purge control circuit. This is the direction-specific complement to P20C6 (circuit low). A high circuit voltage typically indicates a short to the supply voltage on the control wire, an open circuit in the solenoid coil allowing the control wire to float high, or a fault in the ECM's internal pull-down driver for the purge control output.

In a typical low-side driver configuration, the ECM supplies a ground path to activate the solenoid, and the circuit normally sits at low voltage when commanded on. When the ECM reads a high voltage on the control line where it expects low voltage, P20C4 is logged. The air purge function, which clears residual reductant from the dosing injector after shutdown, is compromised and the injector becomes vulnerable to urea crystallization over time.

Diagnosis should include checking the wiring harness for short circuits to the battery positive or ignition-switched supply, inspecting the solenoid for an open coil, and verifying the connector is properly seated. ECM driver health can be evaluated by checking the output pin voltage during a commanded state.

Common causes

Most-frequently reported root causes when P20C4 is logged.

  • 1
    Short circuit to supply voltage (battery positive or switched supply) on the air purge solenoid control wire.
  • 2
    Open circuit in the solenoid coil causing the control wire to float at high voltage.
  • 3
    Damaged wiring harness contacting a live circuit in the engine bay.
  • 4
    Faulty connector with terminal back-out creating an open circuit condition.
  • 5
    ECM internal driver fault holding the output high rather than pulling it low.
  • 6
    Incorrectly wired repair splice on the harness introducing a short to voltage.

Symptoms drivers notice

MIL illuminated.
Reductant system warning message on instrument cluster.
Air purge cycle fails to complete after engine shutdown.
Urea crystallization at dosing injector tip with extended fault duration.
Possible related SCR dosing or efficiency codes if injector blockage develops.

How to diagnose P20C4

A typical diagnostic flow when this code is present.

  1. 1
    Scan and record all DTCs and freeze frame data before beginning electrical testing.
  2. 2
    Visually inspect the air purge solenoid wiring harness for any areas where the harness may contact a voltage source.
  3. 3
    Disconnect the solenoid connector and measure voltage on the control wire with the ignition on; a high reading indicates a short to voltage.
  4. 4
    Measure solenoid coil resistance; an open reading (infinite resistance) means the coil has failed and the wire floats high.
  5. 5
    Inspect the connector for terminal back-out or damaged pins that might cause an open circuit.
  6. 6
    If wiring and solenoid are confirmed good, check ECM output pin behavior during commanded purge activation.
  7. 7
    Repair any identified wiring fault or replace the solenoid as needed, then clear codes and verify repair.

Vehicles where we've handled P20C4

Platforms in our catalogue with confirmed P20C4 coverage.

AUDI A4 20D

Related powertrain codes

Frequently asked questions

How is P20C4 different from P20C3 and P20C6?

P20C3 is a general circuit fault. P20C6 is a low voltage or short-to-ground fault. P20C4 is specifically a high voltage fault indicating a short to supply voltage or an open coil.

Can an open solenoid coil cause a high circuit code?

Yes. In a low-side driver circuit, an open coil removes the current path to ground, causing the control wire to sit at supply voltage, which the ECM interprets as a circuit high fault.

Is P20C4 urgent?

Not immediately for drivability, but leaving it unrepaired will eventually allow urea deposits to block the dosing injector, causing SCR system failure and potential NOx compliance issues.

Could P20C4 be caused by water damage?

Yes. Water ingress can cause short circuits to voltage or corrosion that creates high-resistance open conditions. Inspect connector seals and the harness routing near water entry points.

Disabling P20C4 in software

RaceTune can permanently disable P20C4 — 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 P20C4 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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