P25D5

Reductant Level Sensor Circuit Low Bank 1

P25D5 is a generic OBD-II powertrain diagnostic trouble code: Reductant Level Sensor Circuit Low Bank 1. 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
P25D5
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P25D5 means

P25D5 is set when the control module measures a voltage on the Bank 1 DEF level sensor circuit that is below the expected operating range for an extended period. A continuously low voltage signal typically indicates a short to ground in the sensor wiring, a failed sensor with an internal short, or an open in the sensor reference or supply circuit pulling the signal line low.

The DEF level sensor informs the control module and the driver about the remaining fluid in the reductant tank. A stuck-low signal will usually cause the module to interpret the tank as empty, which can trigger a low DEF warning, inhibit dosing, or generate further SCR system faults. The system may not know whether the tank is genuinely empty or whether the circuit itself is at fault.

Before replacing the sensor, technicians should verify that the DEF tank actually contains fluid, then inspect the wiring for a ground short. If the DEF tank is confirmed full but the sensor reads low, the fault is most likely in the sensor or wiring rather than a genuine empty-tank condition. Comparing signal voltage at the module connector versus the sensor connector can help pinpoint the fault location.

Common causes

Most-frequently reported root causes when P25D5 is logged.

  • 1
    Short to ground in the DEF level sensor signal wire.
  • 2
    Failed DEF level sensor with internal short causing a low output.
  • 3
    Open in the sensor supply or reference voltage circuit pulling signal low.
  • 4
    Corroded connector creating a ground path through moisture.
  • 5
    DEF tank genuinely empty (rule out first before electrical diagnosis).
  • 6
    Faulty PCM or control module input circuit with shorted input.
  • 7
    Wiring harness damage from abrasion exposing the signal wire to ground.

Symptoms drivers notice

MIL illuminated.
DEF level gauge pegged at empty or showing no reading.
Low DEF warning lamp active even with a full tank.
Possible SCR system or emissions fault message.
DEF dosing may be inhibited if the module interprets the signal as an empty tank.

How to diagnose P25D5

A typical diagnostic flow when this code is present.

  1. 1
    Verify the DEF tank level physically before beginning electrical diagnosis.
  2. 2
    Record all DTCs and review freeze-frame data with a scan tool.
  3. 3
    Disconnect the DEF level sensor connector and measure voltage on the signal circuit; a grounded wire will show near 0 V with the sensor disconnected.
  4. 4
    Inspect the sensor harness for damage, chafing, or contact with ground points.
  5. 5
    Measure resistance between the signal wire and chassis ground with the connector unplugged.
  6. 6
    Check sensor supply voltage at the connector with the ignition on.
  7. 7
    Replace the DEF level sensor if wiring checks out and the fault is confirmed to be internal to the sensor.

Vehicles where we've handled P25D5

Platforms in our catalogue with confirmed P25D5 coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Could an empty DEF tank cause P25D5?

Yes. Always check the physical DEF level first. An empty tank can produce a low sensor voltage on some sensor designs.

Will P25D5 trigger a torque de-rate immediately?

Not always immediately, but if the module determines it cannot confirm adequate DEF level for dosing, a de-rate may follow after a defined drive cycle.

Is P25D5 different from P25D4?

Yes. P25D4 is an intermittent signal fault while P25D5 is a continuously low signal. They can share root causes but require different diagnostic approaches.

How do I test the DEF level sensor resistance?

Disconnect the sensor and measure resistance across its terminals per the vehicle service manual specification. Shorted sensors will read very low resistance to ground.

Disabling P25D5 in software

RaceTune can permanently disable P25D5 — 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 P25D5 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 EDC17C50 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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