P29E9

NOx Sensor Circuit Range/Performance - Sensor 2

P29E9 is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit Range/Performance - 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
P29E9
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P29E9 means

P29E9 is the Sensor 2 counterpart to P29E8, set when the downstream NOx sensor (post-SCR catalyst) produces a signal that is electrically valid but does not correlate with the PCM's expected post-catalyst NOx level. The downstream sensor is critical for closed-loop SCR control and for verifying that the catalyst is converting NOx at the required efficiency.

A plausibility fault on the downstream sensor may indicate that the SCR catalyst is aged or poisoned, that DEF dosing is incorrect, or that the sensor itself has drifted. The PCM compares the measured outlet NOx against an expected value based on inlet NOx, DEF injection rate, and catalyst temperature. Divergence beyond a threshold for a sustained time sets the code.

Diagnosis must distinguish between a legitimate catalyst efficiency drop, a dosing system fault sending the wrong DEF quantity, and a sensor accuracy problem. Reviewing DEF consumption rate, catalyst temperature history, and inlet NOx data alongside the outlet reading provides the most efficient diagnostic pathway before any parts are ordered.

Common causes

Most-frequently reported root causes when P29E9 is logged.

  • 1
    SCR catalyst degraded, poisoned, or at end of service life.
  • 2
    Incorrect DEF dosing due to a faulty dosing module or injection valve.
  • 3
    DEF quality issue (diluted or contaminated urea solution) reducing conversion efficiency.
  • 4
    NOx Sensor 2 element contaminated or aged.
  • 5
    Exhaust leak between the SCR catalyst and Sensor 2 introducing ambient air.
  • 6
    Sensor heater fault causing incorrect operating temperature.
  • 7
    DEF deposit build-up in the mixer or catalyst inlet affecting conversion.

Symptoms drivers notice

MIL illuminated, often accompanied by an SCR or emissions warning.
Elevated tail-pipe NOx emissions above legal limits.
Vehicle may derate after several drive cycles if the fault is persistent.
DEF consumption may appear normal despite poor SCR conversion.
Scan tool may show post-catalyst NOx higher than expected relative to pre-catalyst reading.

How to diagnose P29E9

A typical diagnostic flow when this code is present.

  1. 1
    Record all codes and freeze-frame data; note any DEF dosing, DEF quality, or inlet NOx codes.
  2. 2
    Check DEF fluid quality with a refractometer and verify urea concentration is within specification.
  3. 3
    Monitor live inlet (Sensor 1) and outlet (Sensor 2) NOx during a steady-state highway drive.
  4. 4
    Verify DEF dosing quantity matches the commanded injection rate using scan tool data.
  5. 5
    Inspect the exhaust system downstream of the SCR for leaks near Sensor 2.
  6. 6
    Test NOx Sensor 2 heater resistance and supply voltage.
  7. 7
    If DEF, dosing, and wiring checks pass, evaluate SCR catalyst efficiency or replace Sensor 2.

Vehicles where we've handled P29E9

Platforms in our catalogue with confirmed P29E9 coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Could low DEF quality cause P29E9?

Yes. Diluted or contaminated DEF reduces NOx conversion efficiency, which can shift post-catalyst NOx above the expected range and set the rationality fault.

Is P29E9 always a sensor fault?

No. A failing SCR catalyst or incorrect dosing is equally or more likely; always evaluate system performance before assuming the sensor has failed.

Can I clear P29E9 and continue driving?

Clearing the code does not fix the underlying fault; it will return, and persistent SCR non-compliance may trigger a derate or speed limiter.

Do both P29E8 and P29E9 need to be present to indicate SCR failure?

No. Either code alone can point to an SCR system issue; P29E9 alone often indicates a catalyst or dosing problem rather than an upstream sensor issue.

Disabling P29E9 in software

RaceTune can permanently disable P29E9 — 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 P29E9 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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