P24AF

SCR NOx Catalyst Efficiency Below Threshold Bank 1

P24AF is a generic OBD-II powertrain diagnostic trouble code: SCR NOx Catalyst Efficiency Below Threshold Bank 1. 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
P24AF
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on, possible limp mode)
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What P24AF means

P24AF is set when the ECM or aftertreatment module determines that the selective catalytic reduction (SCR) catalyst on Bank 1 is not converting NOx at the minimum required efficiency level. The system typically uses NOx sensors upstream and downstream of the SCR catalyst, along with DEF dosing feedback, to continuously calculate conversion efficiency.

Possible causes span a wide range: inadequate DEF dosing due to a faulty injector or pump, incorrect DEF concentration (diluted or contaminated fluid), a poisoned or thermally damaged SCR catalyst substrate, clogged DEF lines, or a fault in the NOx sensor providing inaccurate efficiency data. The catalyst itself may be degraded by sulphur poisoning, incorrect DEF, or overtemperature events.

Before condemning the SCR catalyst, verify the entire DEF delivery system is functioning correctly, that the DEF fluid meets specification, and that both NOx sensors read plausibly. A faulty downstream NOx sensor reporting elevated NOx is a common cause of spurious efficiency codes and should be checked before replacing the expensive catalyst assembly.

Common causes

Most-frequently reported root causes when P24AF is logged.

  • 1
    DEF dosing fault (injector, pump, or pressure issue) reducing reductant delivery.
  • 2
    Contaminated or diluted DEF not providing adequate urea concentration.
  • 3
    Degraded, poisoned, or thermally damaged SCR catalyst substrate.
  • 4
    Faulty downstream NOx sensor reporting falsely high NOx values.
  • 5
    Clogged DEF supply line or injector tip restricting DEF flow.
  • 6
    DEF system frozen in cold climates preventing dosing.
  • 7
    Faulty upstream NOx sensor skewing the efficiency calculation.
  • 8
    Incorrect SCR inlet temperature due to an exhaust system fault.

Symptoms drivers notice

MIL or SCR/NOx warning lamp illuminated.
Possible engine power derate on vehicles with NOx-related regeneration strategies.
Higher tailpipe NOx emissions.
Possible DEF consumption anomaly (too high or too low).

How to diagnose P24AF

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and read all DTCs; check for companion DEF delivery or NOx sensor codes.
  2. 2
    Verify DEF quality using a refractometer to confirm 32.5% urea concentration.
  3. 3
    Check DEF dosing live data: commanded versus actual injection quantity and DEF pump pressure.
  4. 4
    Inspect the DEF injector for blockage, clogging, or circuit faults.
  5. 5
    Compare upstream and downstream NOx sensor readings against expected values during a drive cycle.
  6. 6
    Perform a NOx sensor plausibility test to rule out a faulty sensor before condemning the catalyst.
  7. 7
    If dosing and sensors are confirmed good, evaluate SCR catalyst replacement as the final step.

Vehicles where we've handled P24AF

Platforms in our catalogue with confirmed P24AF coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can contaminated DEF cause P24AF?

Yes. Diluted or contaminated DEF with urea concentration outside specification cannot reduce NOx effectively, leading to low catalyst efficiency readings.

Is the SCR catalyst expensive to replace?

SCR catalyst assemblies can be costly; always verify the DEF delivery system and NOx sensors are functioning correctly before authorising catalyst replacement.

Can a bad NOx sensor cause P24AF without a real catalyst problem?

Yes. A downstream NOx sensor reading high (reporting more NOx than actually present) will make efficiency appear low; sensor validation is a critical early step.

How do I confirm DEF concentration is correct?

Use an automotive refractometer calibrated for urea solution; the reading should correspond to approximately 32.5% urea; readings well outside this range indicate contaminated or incorrect fluid.

Disabling P24AF in software

RaceTune can permanently disable P24AF — 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 P24AF 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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