P24B4

NOx Sensor Circuit Low Voltage Bank 1 Sensor 1

P24B4 is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit Low Voltage 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
P24B4
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P24B4 means

P24B4 is set when the PCM detects that the signal from the upstream NOx sensor (Bank 1, Sensor 1) has fallen below the minimum expected voltage threshold. This sensor is positioned upstream of the SCR catalyst and reports engine-out NOx concentration to the emissions control strategy.

A low-voltage condition prevents the module from calculating accurate reductant injection quantities, which can cause either under-dosing or over-dosing of diesel exhaust fluid (DEF). Under-dosing leads to excess NOx emissions; over-dosing can result in ammonia slip and SCR catalyst contamination.

Common causes include an open circuit in the signal wire, a short to ground, a failed sensor with degraded output, or water intrusion into the connector. The MIL is illuminated upon detection, and the fault must typically be present across multiple trips before a permanent DTC is recorded. Repairing the underlying circuit fault or replacing the sensor resolves the code in most cases.

Common causes

Most-frequently reported root causes when P24B4 is logged.

  • 1
    Open circuit or broken signal wire in the NOx sensor harness.
  • 2
    Short to ground in the NOx sensor signal circuit.
  • 3
    Failed upstream NOx sensor with degraded output signal.
  • 4
    Water or moisture intrusion into the sensor connector causing signal loss.
  • 5
    Corroded sensor connector terminals increasing circuit resistance.
  • 6
    Faulty NOx sensor control module output.
  • 7
    Damaged sensor element due to exhaust thermal shock or contamination.

Symptoms drivers notice

MIL illuminated.
SCR system operating with inaccurate or defaulted dosing strategy.
Potential increase in tailpipe NOx emissions.
Scan tool showing NOx sensor voltage at or near zero.
Possible concurrent SCR-related fault codes.

How to diagnose P24B4

A typical diagnostic flow when this code is present.

  1. 1
    Record all DTCs and freeze frame data with a scan tool before any repairs.
  2. 2
    Observe live NOx sensor voltage data to confirm it is below the expected operating range.
  3. 3
    Inspect the sensor harness from the sensor to the module for open circuits, breaks, or shorts to ground.
  4. 4
    Check the sensor connector for moisture, corrosion, or bent terminals.
  5. 5
    With the connector disconnected, measure continuity and insulation resistance of the signal wire.
  6. 6
    If the wiring is intact, replace the upstream NOx sensor and retest.
  7. 7
    Clear codes and complete a manufacturer-specified drive cycle to verify the repair.

Vehicles where we've handled P24B4

Platforms in our catalogue with confirmed P24B4 coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Is P24B4 the opposite of P24B3?

Yes. P24B3 indicates a high-voltage fault on the same sensor circuit, while P24B4 indicates a low-voltage fault. Both relate to the upstream NOx sensor signal being outside its normal range.

Can a contaminated sensor cause a low-voltage code?

Yes. Sensor element contamination from oil, coolant, or excessive soot can degrade the electrochemical cell, resulting in a reduced or absent output signal.

How long can the fault be present before the MIL lights?

Most OBD-II calibrations require the condition to be detected on two consecutive drive cycles before permanently illuminating the MIL, though some manufacturers use a single-trip threshold for emissions-critical components.

Will clearing the code fix the problem?

Clearing the code removes the stored DTC but does not repair the underlying fault. The code will return unless the circuit or sensor issue is corrected.

Disabling P24B4 in software

RaceTune can permanently disable P24B4 — 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 P24B4 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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