P24D4

NOx Sensor Circuit Range/Performance Bank 1 Sensor 2

P24D4 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
P24D4
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P24D4 means

P24D4 is recorded when the PCM determines that the downstream NOx sensor (Bank 1, Sensor 2) signal is within the valid voltage range but does not behave as expected relative to SCR system operating conditions and upstream NOx sensor data. This is a rationality or plausibility fault rather than a simple electrical circuit failure.

The control module compares the downstream sensor reading against the upstream sensor value, reductant dosing quantities, catalyst temperature, and expected conversion efficiency. If the downstream reading is implausibly high or low, does not respond correctly to changes in dosing, or diverges unreasonably from the model prediction, P24D4 is set. Common causes include a slow or contaminated sensor element, ammonia slip affecting the electrochemical response, or an aging SCR catalyst producing unpredictable conversion efficiency.

Because this sensor is part of the OBD-II SCR catalyst monitoring system, a confirmed performance fault will illuminate the MIL and typically also prevent the SCR catalyst monitor from completing. This will result in an OBD readiness failure at an emissions inspection. Accurate diagnosis requires evaluation of the entire SCR system in addition to the sensor circuit.

Common causes

Most-frequently reported root causes when P24D4 is logged.

  • 1
    Degraded or aging downstream NOx sensor element with slow response.
  • 2
    Ammonia slip from over-dosing contaminating the downstream sensor.
  • 3
    SCR catalyst aging causing unpredictable conversion and unexpected sensor output.
  • 4
    Exhaust leak between the SCR catalyst and the downstream sensor diluting the sample.
  • 5
    Upstream dosing system fault causing inconsistent reductant delivery.
  • 6
    Sensor element contaminated by oil, soot, or DEF crystallization.
  • 7
    Intermittent sensor heater fault causing operating temperature instability.

Symptoms drivers notice

MIL illuminated.
SCR catalyst monitor not completing readiness cycle.
Possible SCR catalyst efficiency codes stored alongside P24D4.
No significant drivability symptoms in most cases.
Freeze frame showing post-catalyst NOx reading inconsistent with operating conditions.

How to diagnose P24D4

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and freeze frame data; note any upstream or dosing system codes.
  2. 2
    Check live data for both upstream and downstream NOx sensor readings and compare their relationship.
  3. 3
    Verify the SCR reductant dosing system is functioning correctly with no delivery faults.
  4. 4
    Check for exhaust leaks between the SCR outlet and the downstream sensor location.
  5. 5
    Inspect the downstream NOx sensor heater operation and confirm the element reaches operating temperature.
  6. 6
    If no systemic faults are found, replace the downstream NOx sensor and retest.
  7. 7
    Perform a complete drive cycle including conditions that activate SCR catalyst monitoring.

Vehicles where we've handled P24D4

Platforms in our catalogue with confirmed P24D4 coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can P24D4 be caused by over-dosing of DEF?

Yes. Excess ammonia (ammonia slip) exiting the SCR catalyst can react with the downstream NOx sensor element and alter its response, causing a rationality fault.

Does P24D4 mean the SCR catalyst has failed?

Not necessarily. P24D4 is a sensor performance code. A separate catalyst efficiency code would be needed to confirm catalyst failure. Diagnose the sensor first.

Why won't my readiness monitors complete with P24D4 active?

The SCR catalyst monitor requires a functioning downstream NOx sensor to evaluate conversion efficiency. A performance fault on that sensor prevents the monitor from making a valid pass or fail determination.

How do I know if the sensor or the SCR system is causing this code?

Compare upstream and downstream sensor trends during a drive cycle. If the downstream sensor shows no response to changes in dosing or engine load while the upstream sensor responds normally, the downstream sensor is likely faulty.

Disabling P24D4 in software

RaceTune can permanently disable P24D4 — 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 P24D4 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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