P20C6

Reductant Heater D Control Circuit Performance

P20C6 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater D Control Circuit Performance. 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
P20C6
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on)
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What P20C6 means

P20C6 is stored when the ECM detects a low voltage or short-to-ground condition in the control circuit for the reductant injection air purge system. The air purge function uses compressed air to clear residual reductant from the dosing injector and lines after shutdown, preventing crystallization of urea deposits that could block the injector tip.

The purge control circuit typically consists of a solenoid or valve driven by the ECM via a low-side driver. A low circuit fault indicates that the ECM measured a lower-than-expected voltage on the control wire, which can result from a wiring short to ground, a damaged solenoid with shorted coil windings, or an internal driver fault within the control module itself.

When the air purge cycle cannot be confirmed, the ECM flags the circuit fault and illuminates the MIL. Over time, failure to purge the injector can lead to urea crystal buildup, injector clogging, and degraded SCR performance. Diagnosis should begin with a visual inspection of the wiring harness and connector before condemning any component.

Common causes

Most-frequently reported root causes when P20C6 is logged.

  • 1
    Short circuit to ground in the air purge solenoid control wiring.
  • 2
    Damaged or corroded connector at the air purge solenoid or ECM harness plug.
  • 3
    Failed air purge solenoid with shorted internal coil winding.
  • 4
    Chafed wiring harness causing intermittent or permanent short to chassis ground.
  • 5
    Internal ECM driver circuit fault affecting the purge control output.
  • 6
    Water ingress into the reductant module connector causing low-resistance path to ground.

Symptoms drivers notice

MIL illuminated with P20C6 stored.
Reductant system warning on instrument cluster.
Possible urea deposit buildup at dosing injector tip over time.
Air purge cycle does not complete after engine shutdown.
Related SCR efficiency codes may set if injector becomes blocked.

How to diagnose P20C6

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and freeze frame data with a scan tool before any repairs.
  2. 2
    Locate the air purge solenoid and inspect the wiring harness and connector for visible damage, corrosion, or chafing.
  3. 3
    With the circuit unplugged, measure resistance of the solenoid coil and compare to specification.
  4. 4
    Using a multimeter, check for continuity between the control wire and chassis ground with the connector disconnected to identify a wiring short.
  5. 5
    Verify supply voltage and ground integrity at the solenoid connector with the ignition on.
  6. 6
    If wiring and solenoid test within spec, perform ECM pin-out voltage check to evaluate the driver output.
  7. 7
    Repair any wiring faults found, reconnect, clear codes, and verify the purge control circuit functions correctly.

Related powertrain codes

Frequently asked questions

What happens if the air purge does not work?

Without air purging, residual DEF in the injector tip can crystallize and clog the orifice, eventually causing insufficient reductant dosing and NOx efficiency faults.

Is P20C6 a hard fault or can it be intermittent?

It can be either. Intermittent cases are often caused by heat-cycling of a chafed wire or corroded connector. Check connectors carefully even if the fault does not reproduce immediately.

Can I clear P20C6 and monitor if it returns?

Yes, after confirming wiring integrity you may clear the code and observe whether it returns during the post-shutdown purge cycle. Persistent return suggests a solenoid or driver fault.

Does P20C6 affect drivability immediately?

Not usually in the short term. Drivability impact is indirect: if the injector eventually blocks due to failed purging, SCR efficiency drops and torque derates may follow.

Disabling P20C6 in software

RaceTune can permanently disable P20C6 — 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.

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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