P24BC
SCR NOx Catalyst Efficiency Below ThresholdP24BC is a generic OBD-II powertrain diagnostic trouble code: SCR NOx Catalyst Efficiency Below Threshold. 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.
What P24BC means
P24BC is set when the ECM determines that the SCR catalyst is not converting NOx at the efficiency level required to meet emissions standards. The ECM calculates conversion efficiency by comparing NOx concentration values from sensors upstream and downstream of the SCR. When the ratio of NOx reduction falls below a calibrated threshold over a monitored drive cycle, the fault is stored and the MIL is illuminated.
This is a general SCR efficiency code that may be used on single-bank exhaust systems where bank-specific codes such as P24B0 or P24B1 are not applicable. The root cause may lie with the SCR catalyst itself, the DEF dosing system, NOx sensor accuracy, or DEF quality. Catalyst degradation from sulfur poisoning, high-temperature exposure, or physical damage are common sources of genuine efficiency loss.
Emissions regulations in many markets require a torque or speed derate if an SCR efficiency fault is not resolved within a defined distance or time window. Diagnosis should proceed systematically from DEF quality and sensor checks before the catalyst is condemned.
Common causes
Most-frequently reported root causes when P24BC is logged.
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1
SCR catalyst aged or poisoned reducing NOx conversion efficiency below the required threshold.
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2
DEF (AdBlue) contamination or incorrect urea concentration impairing the SCR chemical reaction.
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3
Faulty DEF dosing injector not delivering the correct amount of reagent to the exhaust.
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4
Failed or biased downstream NOx sensor reporting higher NOx than actually present after the SCR.
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5
Failed or biased upstream NOx sensor causing incorrect DEF dosing quantity calculation.
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6
DEF deposit buildup (crystallization) in the exhaust system blocking reagent distribution.
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7
Exhaust system leak between the DEF injection point and the SCR catalyst outlet.
Symptoms drivers notice
How to diagnose P24BC
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and retrieve all DTCs; record freeze-frame and note any related DEF or NOx sensor codes.
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2
Verify DEF fluid quality and urea concentration; replace with certified DEF if any doubt exists.
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3
Check DEF tank level and inspect for contamination or foreign material in the tank.
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4
Review live NOx sensor data upstream and downstream of the SCR at operating temperature to assess actual conversion efficiency.
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5
Test DEF dosing injector operation and inspect for crystallized deposits at the injection point.
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6
Check for exhaust leaks between the DEF injection port and the SCR downstream NOx sensor.
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7
If DEF system and NOx sensors are confirmed functional, evaluate SCR catalyst efficiency data and consider catalyst replacement.
Vehicles where we've handled P24BC
Platforms in our catalogue with confirmed P24BC coverage.
Related powertrain codes
- P2400 — Evaporative Emission System Leak Detection Pump Control Circuit/Open
- P2401 — Evaporative Emission System Leak Detection Pump Control Circuit Low
- P2402 — Evaporative Emission System Leak Detection Pump Control Circuit High
- P2404 — EVAP Leak Detection Pump Sense Circuit: Implausible Signal
- P2405 — Evaporative Emission System Leak Detection Pump Sense Circuit Low
- P2407 — Evaporative Emission System Leak Detection Pump Sense Circuit Intermittent/Erratic
Frequently asked questions
How quickly will a derate occur after P24BC is set?
The derate threshold varies by manufacturer and regional regulation, but many systems allow a set number of engine starts or a defined mileage before enforcing a power reduction. The MIL illuminates immediately.
Does filling with fresh DEF always clear P24BC?
If contaminated DEF was the cause, replacing it with certified fluid and performing a drive cycle may allow the code to clear. If catalyst degradation or a sensor fault is the cause, the code will return.
Can P24BC be caused by a cold engine or short trips?
SCR systems require the catalyst to reach operating temperature to function efficiently. Very short trips in cold conditions may transiently reduce efficiency, but sustained P24BC typically indicates a real system issue.
What is the difference between P24BC and P24B0?
P24B0 specifically references bank 1 on a multi-bank exhaust system, while P24BC is a general SCR efficiency code used on single-bank systems or as a summary code where bank differentiation is not applicable.
Disabling P24BC in software
RaceTune can permanently disable P24BC — 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.
ECUs with a P24BC 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 P24BC in your scan?
Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.
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