P20F4

Reductant Heater A Control Circuit High

P20F4 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater A Control Circuit High. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P20F4
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P20F4 means

P20F4 is set when the SCR or engine control module detects an abnormally high voltage on the control circuit for reductant heater A. This is the counterpart to P22A7 (circuit low) and indicates that the monitored circuit voltage is above the expected upper limit during a commanded heater event or during a circuit rationality check.

A high-voltage condition on a heater control circuit is commonly caused by an open circuit in the heater element or its wiring, a failed relay that is stuck open or not providing a proper current return path, a broken ground connection, or a short to a voltage supply source. When the heater circuit is open, the module may see battery or reference voltage on the sense line where it expects a loaded voltage, triggering this high-side fault.

Diagnosis should begin with a connector and wiring inspection for open circuits, followed by measurement of heater element resistance to check for an open winding. Relay function and ground integrity should also be verified before any module-level diagnosis is performed.

Common causes

Most-frequently reported root causes when P20F4 is logged.

  • 1
    Open circuit in the reductant heater A wiring or harness.
  • 2
    Failed heater element with an open winding presenting high impedance.
  • 3
    Broken or corroded ground connection for the heater circuit.
  • 4
    Stuck-open relay failing to complete the heater circuit.
  • 5
    Short to battery voltage in the heater control wire.
  • 6
    Damaged connector with backed-out or missing terminals.
  • 7
    Internal module driver circuit failure providing incorrect output.

Symptoms drivers notice

MIL illuminated with P20F4 stored.
Reductant heater A does not operate, risking DEF freezing in cold conditions.
SCR or AdBlue warning lamp may be active.
Possible additional reductant system codes stored alongside P20F4.
No immediate driveability symptoms in warm climates.

How to diagnose P20F4

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and retrieve all DTCs and freeze-frame data before any tests.
  2. 2
    Inspect the reductant heater A wiring harness and connector for open circuits, backed-out pins, or corrosion.
  3. 3
    Disconnect the heater connector and measure heater element resistance to check for an open winding.
  4. 4
    Verify the heater ground connection is clean, tight, and properly bonded to chassis.
  5. 5
    Test the heater relay for proper coil resistance and contact continuity in both energized and de-energized states.
  6. 6
    With connector reconnected and heater commanded on, measure voltage on the control circuit to identify a short to supply.
  7. 7
    If all external circuits test within specification, consider module output driver fault and consult reprogramming or replacement procedures.

Vehicles where we've handled P20F4

Platforms in our catalogue with confirmed P20F4 coverage.

AUDI A4 20D
AUDI A6
2015
AUDI A6 30D
2015
AUDI A7
MB GLC220 21D
2017
MB GLE350 30D
AUDI A7 30D

Related powertrain codes

Frequently asked questions

Why would the circuit read high if the heater is broken?

An open heater element or broken wire leaves the sense circuit floating or pulled up to supply voltage through internal module bias resistors, which the module interprets as a high-side fault.

Is P20F4 more serious than P22A7?

Both indicate a heater circuit fault that disables the reductant heater. The severity is similar in terms of system impact; P20F4 points to an open or high-impedance condition rather than a short to ground.

Can a bad relay cause P20F4?

Yes. A stuck-open relay does not allow current to flow, which can result in the module sensing an unexpectedly high voltage on the circuit where a loaded voltage is expected.

Do I need to replace the whole DEF heater module for P20F4?

Not necessarily. The fault is often in the wiring, connector, relay, or heater element itself. A full module replacement is only warranted after all external causes have been ruled out.

Disabling P20F4 in software

RaceTune can permanently disable P20F4 — 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 P20F4 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 EDC17CP57 verified 2 software versions
  • Bosch EDC17C66 verified 1 software version
  • Bosch EDC17C74 verified 1 software version
  • Bosch EDC17CP44 verified 1 software version
  • Bosch MD1CP004 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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