P24B5

NOx Sensor Circuit Range/Performance Bank 1 Sensor 1

P24B5 is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit Range/Performance Bank 1 Sensor 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
P24B5
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P24B5 means

P24B5 indicates that the upstream NOx sensor (Bank 1, Sensor 1) signal is within the measurable voltage range but is not behaving as expected relative to known engine operating conditions. Unlike a simple high or low voltage fault, this code reflects a rationality or plausibility failure where the sensor output does not correlate with other engine parameters such as load, fuel injection quantity, or EGR position.

The PCM compares NOx sensor output against a predictive model derived from fuel delivery, engine speed, intake air mass, and EGR valve position. When the measured value differs significantly from the predicted value and cannot be explained by normal transients, a range or performance fault is recorded. This can result from a slow-responding sensor, a partially contaminated sensing element, or a sensor that has drifted out of calibration.

Sensor aging, sulfur poisoning of the ceramic element, and exhaust leaks upstream of the sensor that dilute the exhaust sample are also recognized causes. The MIL is illuminated after the fault is confirmed, and SCR dosing accuracy may be compromised.

Common causes

Most-frequently reported root causes when P24B5 is logged.

  • 1
    Aging or degraded NOx sensor element with slow response or drift.
  • 2
    Sulfur poisoning of the sensor ceramic element from high-sulfur fuel.
  • 3
    Exhaust leak upstream of the sensor diluting the exhaust gas sample.
  • 4
    Faulty EGR system skewing engine-out NOx levels beyond predicted range.
  • 5
    Intake air system leak causing abnormal air-fuel ratio and NOx output.
  • 6
    Contamination of the sensor element by oil or coolant.
  • 7
    Intermittent NOx sensor heater fault causing incorrect operating temperature.

Symptoms drivers notice

MIL illuminated.
SCR dosing strategy operating with reduced accuracy.
Possible marginal or failing emissions test results.
No obvious drivability complaint in many cases.
Freeze frame showing NOx sensor reading inconsistent with engine load.

How to diagnose P24B5

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and freeze frame data; look for accompanying EGR, intake, or exhaust codes.
  2. 2
    Review live data to compare actual NOx sensor output against engine load and fuel delivery.
  3. 3
    Inspect for exhaust leaks upstream of the NOx sensor that could dilute the sample.
  4. 4
    Check NOx sensor heater current and temperature to confirm the element is at operating temperature.
  5. 5
    Evaluate EGR system operation for faults that might cause abnormal engine-out NOx.
  6. 6
    If no external causes are found, replace the NOx sensor and retest.
  7. 7
    Perform a complete drive cycle and monitor sensor correlation with engine parameters.

Related powertrain codes

Frequently asked questions

Why does P24B5 set if the sensor voltage looks normal on the scan tool?

P24B5 is a rationality fault. The voltage may be within bounds, but the sensor reading does not match what the PCM expects given the current engine conditions.

Can using low-quality or high-sulfur diesel cause this code?

Yes. Sulfur in diesel fuel can poison the NOx sensor ceramic element, causing it to read inaccurately even though it appears electrically healthy.

Is P24B5 emissions-test critical?

Yes. A stored or pending P24B5 will generally cause an OBD readiness failure and result in a failed emissions inspection.

Does an exhaust leak always cause visible symptoms?

Not always. A small exhaust leak upstream of the sensor may not produce audible noise or obvious fumes but can still dilute the exhaust sample enough to trigger a rationality fault.

Disabling P24B5 in software

RaceTune can permanently disable P24B5 — 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.

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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