P2579

NOx Sensor Circuit Range/Performance (Bank 1 Sensor 2)

P2579 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
P2579
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
Need P2579 disabled?
RaceTune permanently disables any OBD-II trouble code on supported ECUs — for motorsport, off-road, and export use.
→

What P2579 means

P2579 is set when the downstream NOx sensor (Bank 1, Sensor 2, located after the SCR catalyst) produces a signal that is within the electrical operating range but does not match expected values based on operating conditions and upstream sensor data. This is a plausibility or performance fault rather than a hard electrical failure.

The downstream NOx sensor is critical for closed-loop SCR efficiency monitoring. The ECM compares upstream and downstream NOx readings to calculate SCR conversion efficiency. If the downstream sensor reading is implausibly high or low relative to the upstream reading and the known reductant dosing rate, a performance fault is set. This can indicate a failed or contaminated sensor, SCR catalyst degradation, or incorrect reductant dosing.

A performance fault on the downstream NOx sensor can mask genuine SCR system problems, since the ECM relies on this sensor to detect catalyst failure and reductant quality issues. The MIL is illuminated and the fault is logged. Corrective action should be taken promptly to restore closed-loop NOx control and avoid masking other emissions system faults.

Common causes

Most-frequently reported root causes when P2579 is logged.

  • 1
    Contaminated or aged downstream NOx sensor producing inaccurate readings.
  • 2
    SCR catalyst degradation resulting in low NOx conversion efficiency.
  • 3
    Incorrect or contaminated DEF fluid causing under-dosing or over-dosing.
  • 4
    Reductant dosing system fault (pump, injector) affecting actual SCR inlet concentration.
  • 5
    Exhaust leak between the SCR catalyst and the downstream sensor.
  • 6
    High-resistance connection in the NOx sensor circuit causing signal attenuation.
  • 7
    Failed NOx sensor control module or sensor heater affecting sensor accuracy.

Symptoms drivers notice

MIL illuminated with P2579 stored.
SCR system efficiency monitoring compromised.
NOx emissions may be elevated above regulatory limits.
Scan tool shows downstream NOx reading implausibly high or not tracking upstream data.
Other SCR-related DTCs may accompany P2579 if the root cause affects the broader system.

How to diagnose P2579

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and log all DTCs along with live upstream and downstream NOx sensor values.
  2. 2
    Verify DEF quality and concentration with a refractometer.
  3. 3
    Check reductant dosing system operation (injector, pump) for correct operation.
  4. 4
    Inspect the exhaust system between the SCR outlet and the downstream sensor for leaks.
  5. 5
    Check the NOx sensor connector and wiring for high-resistance connections.
  6. 6
    Compare upstream vs downstream NOx readings at steady cruise conditions to estimate SCR efficiency.
  7. 7
    If SCR conversion efficiency is confirmed low, inspect the catalyst before replacing the sensor.

Vehicles where we've handled P2579

Platforms in our catalogue with confirmed P2579 coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Does P2579 always mean the NOx sensor is bad?

No. A degraded SCR catalyst or incorrect DEF quality can produce a performance fault on a perfectly functional sensor.

Can I clear P2579 and pass an emissions test?

Clearing the code will turn off the MIL temporarily, but it will return if the underlying fault persists; the vehicle is unlikely to pass a proper emissions inspection.

How do I know if the SCR catalyst has failed vs the sensor?

Compare upstream and downstream NOx values at steady cruise; if both sensors read similarly (low conversion efficiency), the catalyst is suspect. If the upstream sensor reads normally but downstream is erratic, the sensor is more likely at fault.

Is P2579 related to DEF quality?

Yes. Contaminated or diluted DEF reduces SCR efficiency, which can cause the downstream NOx reading to exceed expected thresholds and set a performance code.

Disabling P2579 in software

RaceTune can permanently disable P2579 — 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 P2579 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.

Got P2579 in your scan?

Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.

Upload your file