P0408

Exhaust Gas Recirculation Sensor B Circuit High

P0408 is a generic OBD-II powertrain diagnostic trouble code: Exhaust Gas Recirculation Sensor B Circuit High. It is logged by the engine control unit when the egr monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0408
Group
Powertrain
System
EGR
Severity
Warning (MIL on)
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What P0408 means

P0408 — "Exhaust Gas Recirculation (EGR) Sensor \"B\" Circuit High" — is stored when the ECM detects that the voltage signal from the secondary EGR valve position sensor \"B\" exceeds the maximum threshold of the expected operating range, typically above approximately 4.5V on a 5V-referenced system. It is the mirror fault to P0407 (sensor B circuit low) and the sensor \"B\" counterpart to P0406 (sensor \"A\" circuit high). P0408 appears on vehicles fitted with dual-position-sensor EGR valves — principally Toyota 1GD-FTV and 2GD-FTV diesel platforms, select Ford diesel applications, Mazda SH-VPTS engines, and European vehicles using Bosch or Continental dual-feedback high-pressure EGR valves.

The electrical causes for a high-circuit condition on sensor B are the same as for P0406 on sensor A: a short between the signal wire and the 5V reference conductor, a short to 12V battery voltage, an open return (ground) path that leaves the ECM input floating high via a pull-up resistor, or an internally failed sensor B element that outputs maximum voltage regardless of valve position. Because sensor \"A\" is typically still providing valid feedback when only P0408 is set, the ECM may maintain a degree of closed-loop EGR control using sensor A, limiting the immediate drivability impact. The ECM will nevertheless illuminate the MIL and disable full dual-sensor plausibility checking, which can mask developing faults on sensor A.

When P0408 is stored alongside P0406, both EGR position sensors are simultaneously reading high. This pattern most strongly suggests a shared wiring fault — a common signal-to-reference short, a shared connector pin bridge, or a common reference circuit over-voltage — rather than two independently failed sensor elements. A systematic wiring inspection of the dual-sensor connector and harness is the appropriate starting point.

Common causes

Most-frequently reported root causes when P0408 is logged.

  • 1
    Short to the 5V reference voltage on the sensor \"B\" signal wire — insulation damage causing the signal and reference conductors to contact each other.
  • 2
    Short to battery voltage (12V) on the sensor \"B\" signal wire.
  • 3
    Open circuit in the sensor \"B\" signal return (ground) path — ECM pull-up resistor forces the input high, causing P0408 from a broken wire.
  • 4
    Failed EGR position sensor \"B\" with an internal fault driving the potentiometer output to maximum voltage regardless of valve position.
  • 5
    Corroded or bridged connector pins between the sensor \"B\" signal pin and the reference or supply pin.
  • 6
    EGR valve mechanically stuck in the fully-open position, with sensor B correctly reporting maximum voltage for that position.
  • 7
    Incorrectly repaired or spliced wiring connecting the sensor \"B\" signal line to a voltage source.
  • 8
    PCM input circuit failure on the sensor \"B\" channel holding the input high (uncommon — only after external components are ruled out).

Symptoms drivers notice

Malfunction Indicator Lamp (MIL / Check Engine Light) illuminates.
Rough idle or hesitation if the ECM interprets the high B-signal as the valve being fully open and adjusts EGR control accordingly.
Reduced fuel economy and higher tailpipe emissions from degraded EGR modulation.
Failed emissions test due to elevated NOx if the EGR system cannot maintain correct exhaust-gas dilution of the intake charge.
Engine stumble or stall at idle in severe cases where the ECM falls back to an aggressive EGR default strategy.
Possible companion code P0406 (sensor A high) if a shared wiring fault drives both sensor signals high simultaneously.

How to diagnose P0408

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and retrieve all stored and pending codes; note whether P0406 (sensor A high) is also present, which would indicate a shared reference or connector fault.
  2. 2
    With the ignition on and engine off, monitor live sensor \"B\" voltage — a reading consistently above 4.5V confirms the high-circuit condition; compare against live sensor \"A\" voltage.
  3. 3
    With the sensor connector unplugged, measure voltage on the sensor \"B\" signal wire at the harness side — any voltage present (should be near 0V on most systems with connector removed) confirms a short to voltage in the harness.
  4. 4
    Inspect the dual-sensor EGR connector for bridged pins between the signal and reference pins; check for moisture-induced conductive contamination across the connector face.
  5. 5
    Verify 5V reference line integrity; if the reference wire has shorted to the sensor \"B\" signal wire, both will read abnormally.
  6. 6
    Check sensor \"B\" signal return (ground) path continuity — an open ground can produce a high-input reading via the PCM pull-up; measure resistance from ground pin to chassis.
  7. 7
    Disconnect the sensor and measure sensor \"B\" potentiometer resistance between signal and ground pins across valve travel range — open circuit (infinite resistance) with a PCM pull-up in circuit confirms a failed sensor.
  8. 8
    Inspect the EGR valve for a mechanically stuck-open condition; if the valve cannot be moved closed, address the mechanical fault before condemning the sensor circuit.

Related powertrain codes

Frequently asked questions

What is the difference between P0407 and P0408?

P0407 means EGR position sensor \"B\" signal voltage is below the lower limit — indicating a short to ground, open ground, or sensor outputting near zero. P0408 means the sensor \"B\" signal voltage is above the upper limit — indicating a short to reference, open return path (with pull-up), or sensor outputting near maximum. Both affect the same sensor on the same valve; they describe opposite directions of signal failure.

Can an open-circuit wire cause P0408 instead of P0407?

Yes — if the PCM input for sensor \"B\" uses an internal pull-up resistor to the reference rail, a broken signal wire leaves the input floating high, and the ECM reads a continuous high voltage and sets P0408 rather than P0407. This is the same pull-up mechanism that makes an open wire produce P0406 on sensor A. Always check for open circuits, not just shorts, when diagnosing high-circuit codes.

Is P0408 serious enough to stop driving?

Not immediately critical — the vehicle is typically drivable, and on dual-sensor EGR systems the ECM can often continue approximate EGR control using sensor A. However, continued operation risks emissions non-compliance, degraded EGR system performance, and — if the underlying cause is moisture in the connector — progressive corrosion damage to the ECM input circuit. Prompt repair is recommended.

P0406 and P0408 appeared together. What does that indicate?

Simultaneous high-circuit faults on both sensor A and sensor B strongly point to a shared electrical fault: a short between the signal wires and the common reference conductor, a bridged connector, or a reference circuit over-voltage affecting both sensor inputs at once. Independent failure of two separate sensor elements producing high outputs simultaneously is statistically unlikely. Begin diagnosis at the dual-sensor connector and the shared 5V reference circuit.

Disabling P0408 in software

RaceTune can permanently disable P0408 — 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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