P255B

NOx Sensor Circuit Range/Performance Bank 1 Sensor 2

P255B is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit Range/Performance 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
P255B
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P255B means

P255B is stored when the PCM/ECM detects that the signal from the downstream NOx sensor (Bank 1, Sensor 2 - located after the SCR catalyst) is within the circuit's electrical limits but does not correlate with expected values given current operating conditions. The sensor is electrically connected but its output is implausible relative to engine load, exhaust temperature, DEF dosing activity, and catalyst conversion efficiency models.

The downstream NOx sensor is a key feedback element for the Selective Catalytic Reduction system. Its readings are used to verify that the SCR catalyst is converting NOx at the expected efficiency and to adjust urea (AdBlue/DEF) dosing accordingly. When the signal drifts outside the expected performance window, closed-loop SCR control is impaired and NOx slip may increase beyond legal limits.

Common causes include sensor contamination from exhaust soot or sulfur deposits, DEF crystallisation on the sensor tip, wiring harness chafing, or an aged sensor whose electrochemical cell has degraded. The fault may be intermittent at first and become permanent as the sensor degrades further.

Common causes

Most-frequently reported root causes when P255B is logged.

  • 1
    Degraded or contaminated downstream NOx sensor (Bank 1 Sensor 2) with electrochemical cell wear.
  • 2
    DEF (AdBlue) crystallisation or soot deposits on the NOx sensor tip altering its response.
  • 3
    SCR catalyst severely degraded, causing actual NOx output to be far outside modelled range.
  • 4
    Wiring harness damage, corrosion, or intermittent open/short between sensor and PCM.
  • 5
    Exhaust leaks upstream of the sensor introducing false air and distorting readings.
  • 6
    Faulty PCM/ECM NOx sensor input circuit.
  • 7
    Incorrect sensor fitted (wrong part number or incompatible aftermarket unit).

Symptoms drivers notice

MIL (Check Engine Light) illuminated.
SCR system operating in open-loop or degraded mode with increased NOx emissions.
Possible increased DEF consumption if ECM over-doses to compensate.
Possible torque reduction or regeneration inhibition depending on calibration.
No perceptible driveability change in mild cases; possible limp mode on stricter calibrations.

How to diagnose P255B

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and record all stored DTCs and freeze-frame data before clearing anything.
  2. 2
    Read live NOx sensor data from both upstream and downstream sensors and compare with expected values at idle and under load.
  3. 3
    Inspect the NOx sensor connector and wiring for corrosion, chafing, or pushed-back terminals; repair as needed.
  4. 4
    Visually inspect the sensor tip for DEF crystallisation, soot fouling, or physical damage; clean or replace if contaminated.
  5. 5
    Check the SCR catalyst temperature and DEF dosing system for correct operation to rule out catalyst degradation as root cause.
  6. 6
    If wiring and DEF system check out, replace the downstream NOx sensor and retest.
  7. 7
    After repair, perform a drive cycle that includes highway-speed operation to allow SCR closed-loop readiness to complete.

Vehicles where we've handled P255B

Platforms in our catalogue with confirmed P255B coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can I drive with P255B active?

Short-term driving is generally possible, but NOx emissions may exceed legal limits and continued operation can lead to further SCR system faults or torque reduction.

Is the upstream or downstream sensor faulty?

P255B specifically targets Bank 1 Sensor 2, which is the downstream (post-SCR) sensor. The upstream sensor has its own dedicated code set.

Will cleaning the sensor fix it?

If the fault is caused by DEF crystallisation or light soot fouling, careful cleaning may restore operation. Electrochemically degraded sensors must be replaced.

Does this code always mean the sensor needs replacing?

Not always. A degraded SCR catalyst, exhaust leak, or faulty wiring can produce the same code. Always verify the root cause before replacing the sensor.

Disabling P255B in software

RaceTune can permanently disable P255B — 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 P255B 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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