P04B4

EGR Temperature Sensor B Circuit Low

P04B4 is a generic OBD-II powertrain diagnostic trouble code: EGR Temperature Sensor B Circuit Low. 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
P04B4
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P04B4 means

P04B4 is stored when the ECM detects a voltage signal from EGR temperature sensor B that is below the expected minimum threshold. EGR temperature sensors monitor the temperature of exhaust gases passing through the EGR system to allow precise control of EGR flow and to protect components from thermal overload.

A low circuit condition typically indicates a short to ground in the sensor signal wire, an internally shorted sensor, or an open reference voltage circuit. Because the ECM uses this temperature data for EGR duty cycle calculations, a faulty reading can cause the system to restrict or disable EGR operation, which may increase NOx emissions or trigger emission-related limp strategies.

Before replacing the sensor, technicians should verify the reference voltage and ground supply at the sensor connector, inspect the wiring for chafing or pinched insulation causing a short to ground, and confirm the fault is not intermittent due to heat soak or vibration. This code is most common on diesel engines with cooled EGR systems.

Common causes

Most-frequently reported root causes when P04B4 is logged.

  • 1
    EGR temperature sensor B internally shorted or failed.
  • 2
    Signal wire shorted to ground between the sensor and ECM.
  • 3
    Open in the 5V reference supply wire to the sensor.
  • 4
    Damaged or corroded sensor connector causing a short.
  • 5
    Coolant or oil contamination inside the sensor connector.
  • 6
    Wiring harness chafed against engine components creating a ground path.
  • 7
    ECM reference voltage circuit fault.

Symptoms drivers notice

MIL illuminated.
Possible EGR system shutdown or reduced EGR activity.
Increased exhaust smoke on diesel applications.
Reduced engine performance if EGR limp strategy is active.
Elevated NOx emissions.

How to diagnose P04B4

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and note any additional EGR or temperature sensor codes.
  2. 2
    Inspect the EGR temperature sensor B connector for corrosion, moisture, or damaged pins.
  3. 3
    Check for 5V reference voltage at the sensor connector signal pin with the key on.
  4. 4
    Measure sensor signal voltage; a reading near 0V or below 0.1V suggests a short to ground.
  5. 5
    Disconnect the sensor and recheck the signal voltage; if it rises to reference level, the sensor is shorted internally.
  6. 6
    Inspect the signal wire along its routing for chafing or pinch points.
  7. 7
    Replace the sensor only after confirming the wiring harness is intact.

Related powertrain codes

Frequently asked questions

Can a dirty EGR cooler cause P04B4?

Indirectly yes. A heavily clogged EGR cooler can cause abnormal temperature readings that stress the sensor, but P04B4 is an electrical circuit fault rather than a performance fault.

Is P04B4 diesel-specific?

While more common on diesel engines with cooled EGR systems, this code can appear on gasoline engines that use EGR temperature monitoring for precise control.

What does 'circuit low' mean?

It means the voltage on the sensor signal circuit is below the minimum expected value, which usually indicates a short to ground or an open reference voltage supply.

Can I clear P04B4 without repairs?

The code will return if the underlying electrical fault persists. Clearing without repair is not a solution and will not restore correct EGR temperature monitoring.

Disabling P04B4 in software

RaceTune can permanently disable P04B4 — 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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