P20C0

Reductant Heater B Control Circuit High

P20C0 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater B 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
P20C0
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on)
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What P20C0 means

P20C0 indicates that the ECM has detected a higher-than-expected voltage or current on the reductant heater control circuit. The reductant heater is responsible for thawing frozen DEF fluid in cold climates, as aqueous urea solution freezes at approximately -11 degrees Celsius. Without a functioning heater, the SCR system cannot inject reductant and NOx emissions rise significantly.

A high circuit condition typically points to a short to battery voltage in the heater control wiring, a failed heater relay stuck in the energised position, or an open in the return path that forces the ECM to see elevated voltage on its sense line. The ECM logs this code when the measured signal exceeds the high threshold defined in its calibration for a set period.

While the vehicle may continue to operate normally in warm conditions where DEF heating is unnecessary, the fault will prevent cold-weather SCR operation and can lead to DEF pump cavitation if fluid remains frozen. Some vehicles will illuminate the MIL and post an SCR warning without immediately derating.

Common causes

Most-frequently reported root causes when P20C0 is logged.

  • 1
    Short circuit to battery voltage in the reductant heater control wiring.
  • 2
    Failed heater relay providing uncontrolled voltage to the circuit.
  • 3
    Open circuit in the heater ground return path, causing a floating high signal.
  • 4
    Damaged wiring insulation allowing contact with a 12V source.
  • 5
    Faulty ECM heater driver output stage.

Symptoms drivers notice

MIL illuminated with SCR or AdBlue system warning.
Reductant heater not cycling correctly as reported by scan tool live data.
DEF fluid may remain frozen in cold ambient temperatures, disabling SCR.
Possible blown fuse in the reductant heater circuit.

How to diagnose P20C0

A typical diagnostic flow when this code is present.

  1. 1
    Record all codes and freeze frame data with a scan tool before any repairs.
  2. 2
    Inspect the reductant heater fuse and relay; check relay contacts for welding.
  3. 3
    Measure voltage at the heater control circuit connector with the heater commanded off; voltage should be near zero.
  4. 4
    Inspect wiring harness from the ECM to the heater element for shorts to battery voltage.
  5. 5
    Check ground return path continuity for the heater circuit.
  6. 6
    If wiring is intact, perform ECM heater driver test and compare output to specification.

Related powertrain codes

Frequently asked questions

Is P20C0 more common in winter?

It is often discovered in cold weather when the heater is first activated, but the underlying wiring or relay fault can exist year-round.

Can a blown fuse cause P20C0?

A blown fuse typically causes a low or open circuit code; a stuck relay or wiring short to power is the more likely cause of a high circuit fault.

Will the vehicle fail an emissions test with P20C0?

Yes, an active MIL will cause an automatic failure in most OBD-II emissions inspection programs.

Can I clear P20C0 without repair?

The code will return within a short drive cycle if the underlying electrical fault is not corrected.

Disabling P20C0 in software

RaceTune can permanently disable P20C0 — 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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