P255C

NOx Sensor Circuit Low Voltage Bank 1 Sensor 2

P255C is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit Low Voltage Bank 1 Sensor 2. 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
P255C
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P255C means

P255C is triggered when the PCM/ECM measures a voltage on the NOx sensor signal circuit for Bank 1 Sensor 2 (downstream of the SCR catalyst) that falls below the minimum threshold defined in the calibration. The low voltage condition indicates either a short to ground in the signal wiring, an open in the sensor supply or heater circuit, or an internally failed sensor producing no output.

The downstream NOx sensor operates on a principle similar to a wideband oxygen sensor, generating a signal that the ECM interprets as a NOx concentration. When that signal collapses to a near-zero voltage, the ECM cannot use it for SCR feedback and sets this fault. The SCR dosing system will fall back to open-loop or minimum-dose operation.

Diagnosis should start with a careful wiring inspection before condemning the sensor, as harness damage is a frequent cause in high-heat under-floor routing. A sensor heater circuit failure can also indirectly cause a low-signal condition if the sensing element cannot reach operating temperature.

Common causes

Most-frequently reported root causes when P255C is logged.

  • 1
    Short to ground on the NOx sensor signal wire between sensor and ECM.
  • 2
    Open circuit in the sensor reference voltage or supply line.
  • 3
    Failed NOx sensor heater preventing the sensing element from reaching operating temperature.
  • 4
    Corroded or water-damaged sensor connector causing high resistance or a ground path.
  • 5
    Internally failed NOx sensor with collapsed electrochemical cell output.
  • 6
    Wiring harness chafing against exhaust or chassis components causing intermittent short.
  • 7
    ECM connector pin pushed back or corroded on the NOx input channel.

Symptoms drivers notice

MIL illuminated with P255C stored.
SCR system running in open-loop or minimum DEF dose mode.
Potential NOx emissions non-compliance.
Possible additional heater circuit fault codes stored alongside P255C.
No significant driveability change unless additional limp-mode calibration is triggered.

How to diagnose P255C

A typical diagnostic flow when this code is present.

  1. 1
    Read all stored DTCs and record freeze-frame; note any accompanying heater circuit codes.
  2. 2
    With ignition on and engine off, check reference voltage supply to the NOx sensor connector using a multimeter.
  3. 3
    Inspect the sensor harness from the connector to the underbody bulkhead for chafing, heat damage, or pinches.
  4. 4
    Measure signal wire voltage at the ECM connector and compare with sensor connector to isolate a wiring fault.
  5. 5
    If wiring checks pass, test the sensor heater resistance against specification to confirm the heater is functional.
  6. 6
    Replace the downstream NOx sensor if wiring and heater circuit tests are within specification.
  7. 7
    Clear codes and perform a complete drive cycle to confirm the repair.

Vehicles where we've handled P255C

Platforms in our catalogue with confirmed P255C coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can a bad heater cause P255C?

Yes. If the heater fails, the sensor element stays cold and may output a near-zero signal, which the ECM interprets as a low-voltage fault.

Is P255C different from P255B?

Yes. P255B is a range/performance fault (signal present but implausible), while P255C indicates the signal voltage is below the minimum electrical threshold.

How do I check for a short to ground?

Disconnect the sensor, then measure resistance from the signal wire to chassis ground. A reading near zero ohms indicates a short that must be traced in the harness.

Will the vehicle fail an emissions test with this code?

Almost certainly yes, as P255C sets the MIL and indicates SCR monitoring is compromised.

Disabling P255C in software

RaceTune can permanently disable P255C — 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 P255C 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
  • Bosch EDC17CP09 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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