P2F51

Reductant Tank Temperature Sensor Circuit High

P2F51 is a generic OBD-II powertrain diagnostic trouble code: Reductant Tank Temperature Sensor 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
P2F51
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P2F51 means

P2F51 is set when the control module reads a voltage on the reductant (DEF) tank temperature sensor signal circuit that is above the expected maximum operating range. Temperature sensors of this type typically use a negative temperature coefficient thermistor: as temperature increases, resistance decreases and signal voltage falls. A signal voltage above the maximum threshold therefore usually indicates an open circuit or a loss of the sensor ground reference rather than an actual extreme temperature condition.

Without a valid DEF tank temperature reading, the control module cannot determine whether the DEF is frozen or at operating temperature. This impairs the heater control strategy and may prevent DEF dosing until a valid temperature is confirmed. The SCR system's cold-start strategy depends on tank temperature data to manage the heater and dosing schedule.

Diagnosis begins with checking the signal voltage at the sensor connector versus expected values at known ambient temperature. Disconnect the sensor and observe whether the signal rises to near reference voltage, confirming an open in the sensor or signal wire. Also verify ground circuit integrity at the sensor connector.

Common causes

Most-frequently reported root causes when P2F51 is logged.

  • 1
    Open or broken signal wire between the sensor and the control module.
  • 2
    Failed DEF tank temperature sensor with open internal resistance.
  • 3
    Lost or high-resistance ground at the sensor.
  • 4
    Corroded sensor connector causing a loss of electrical contact.
  • 5
    Damaged wiring harness near the DEF tank from road debris or corrosion.
  • 6
    Faulty control module reference voltage or input circuit.
  • 7
    Sensor connector filled with DEF fluid disrupting the electrical connection.

Symptoms drivers notice

MIL illuminated.
DEF tank temperature reading absent or at maximum value on scan tool.
Possible inappropriate DEF heater operation.
SCR system may restrict dosing in cold conditions due to unknown tank temperature.
No direct drivability impact in typical ambient temperatures.

How to diagnose P2F51

A typical diagnostic flow when this code is present.

  1. 1
    Read live data on a scan tool and note the DEF tank temperature sensor value; an implausible maximum reading supports P2F51.
  2. 2
    Inspect the DEF tank temperature sensor connector for corrosion, DEF contamination, or broken pins.
  3. 3
    Disconnect the sensor and measure the signal voltage at the harness connector; near-reference voltage confirms open signal or ground.
  4. 4
    Measure resistance of the sensor element at the sensor terminals across a range of temperatures if possible.
  5. 5
    Check ground circuit continuity from sensor connector to chassis.
  6. 6
    Inspect the wiring harness between sensor and module for open conductors.
  7. 7
    Replace the sensor if confirmed faulty; repair harness faults as found.

Vehicles where we've handled P2F51

Platforms in our catalogue with confirmed P2F51 coverage.

BMW 530XD
2017
BMW X5
2018–2019
BMW 530XD 30D

Related powertrain codes

Frequently asked questions

Can a frozen DEF tank cause P2F51?

P2F51 specifically means the sensor circuit voltage is too high (open circuit type fault), not that the tank is frozen. A frozen tank would cause system performance codes rather than a circuit high fault.

How do I know if the sensor or the wiring is at fault?

Disconnect the sensor connector. If signal voltage rises to near the reference supply voltage, the fault is in the sensor (open internal element) or the ground wire. If voltage was already high before disconnection, suspect the harness.

Does P2F51 affect the tank heater?

It can impair heater control because the module uses tank temperature data to decide when and how long to run the heater.

Is this sensor replaceable separately from the tank?

On many platforms the temperature sensor is integrated into the DEF pump/sender module inside the tank, requiring module replacement. On others it may be a separate serviceable sensor; consult the vehicle service manual.

Disabling P2F51 in software

RaceTune can permanently disable P2F51 — 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 P2F51 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 MD1CP002 verified 1 software version
  • Bosch MD1CS001 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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