P24B0

SCR NOx Catalyst Efficiency Below Threshold Bank 1

P24B0 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
P24B0
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P24B0 means

P24B0 is set when the ECM detects that the selective catalytic reduction (SCR) catalyst on bank 1 is not reducing NOx emissions to the required level. The ECM evaluates SCR efficiency by comparing NOx concentration readings from sensors positioned upstream and downstream of the SCR catalyst. When the conversion efficiency falls below a calibrated threshold, this fault is logged.

SCR systems inject diesel exhaust fluid (DEF/AdBlue) into the exhaust stream to react with NOx over the catalyst. Efficiency can degrade due to catalyst aging or poisoning, insufficient DEF dosing, contaminated DEF, or a failing DEF injection system. NOx sensor accuracy also plays a key role, as a biased downstream NOx sensor can falsely indicate low conversion efficiency.

P24B0 is an emissions-related fault that will illuminate the MIL and may trigger a countdown to a vehicle speed or engine torque limitation if not resolved within the regulatory derate distance or time window. Prompt diagnosis of the DEF dosing system and NOx sensors is recommended before assuming catalyst failure.

Common causes

Most-frequently reported root causes when P24B0 is logged.

  • 1
    SCR catalyst degraded or poisoned by sulfur, oil, or coolant contamination reducing NOx conversion efficiency.
  • 2
    Insufficient DEF dosing due to a faulty DEF injector, dosing pump, or blocked DEF line.
  • 3
    Contaminated or diluted DEF fluid with incorrect urea concentration affecting the SCR reaction.
  • 4
    Faulty downstream NOx sensor providing a biased or inaccurate reading that falsely suggests low efficiency.
  • 5
    Faulty upstream NOx sensor causing incorrect DEF dosing calculation.
  • 6
    DEF deposit buildup in the exhaust mixer or SCR inlet reducing reagent distribution uniformity.
  • 7
    Exhaust system air leak between the DEF injector and SCR catalyst diluting the DEF/exhaust mixture.

Symptoms drivers notice

MIL illuminated; possible separate DEF or emissions warning light.
Potential vehicle speed or torque derate after an extended fault period as required by emissions regulations.
No immediate drivability change in early fault stages.
DEF consumption may be abnormally low if the dosing system is faulty.
Elevated NOx emissions detectable during emissions testing.

How to diagnose P24B0

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and record all active and pending DTCs including any NOx sensor, DEF system, or SCR-related codes.
  2. 2
    Check DEF fluid quality and concentration; replace with fresh certified DEF if in doubt.
  3. 3
    Monitor upstream and downstream NOx sensor values and DEF dosing rate in live data.
  4. 4
    Inspect the DEF injector for deposits or blockage and verify dosing pump operation.
  5. 5
    Test NOx sensor function by comparing upstream and downstream readings against known-good references.
  6. 6
    Check for exhaust system leaks between the DEF injection point and the SCR outlet.
  7. 7
    If DEF system and sensors are functioning correctly, assess SCR catalyst condition and efficiency through live data before considering replacement.

Related powertrain codes

Frequently asked questions

Will my vehicle go into limp mode immediately when P24B0 is set?

Not immediately. Most manufacturers implement a countdown or grace period before enforcing a torque or speed derate. The MIL will illuminate first.

Can contaminated DEF cause P24B0?

Yes. DEF with incorrect urea concentration, contamination, or degradation from exposure to heat or freezing can significantly reduce SCR efficiency.

How do I test if a NOx sensor is faulty?

Compare live upstream and downstream NOx readings with the engine at operating temperature. A downstream reading nearly identical to upstream after the SCR warm-up period suggests low efficiency or a failed catalyst, while an upstream reading that does not correlate with engine load suggests a faulty upstream sensor.

Is SCR catalyst replacement always necessary for P24B0?

No. The fault should first be traced to DEF quality, dosing system integrity, and NOx sensor accuracy before the catalyst is condemned, as these are more common and less expensive causes.

Disabling P24B0 in software

RaceTune can permanently disable P24B0 — 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 P24B0 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 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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