P0406
Exhaust Gas Recirculation Sensor A Circuit HighP0406 is a generic OBD-II powertrain diagnostic trouble code: Exhaust Gas Recirculation Sensor A 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.
What P0406 means
P0406 — "Exhaust Gas Recirculation (EGR) Sensor \"A\" Circuit High" — is the mirror-image fault to P0405. It is stored when the PCM/ECM detects that the voltage signal from the primary EGR valve position sensor \"A\" is above the maximum threshold of the expected operating range — typically above approximately 4.5V on a 5V-referenced sensor circuit. Where P0405 indicates signal collapse (short to ground or open ground), P0406 indicates signal saturation at or near the reference voltage level.
The high-circuit condition arises when the sensor signal conductor is shorted to the 5V reference wire, shorted to a 12V source, when the signal return (ground) path is open (breaking the voltage-divider effect of the potentiometer and leaving the signal pin pulled up to reference), or when the sensor's internal potentiometer element has failed in a way that outputs the maximum voltage regardless of valve position. On some configurations, an open circuit in the sensor harness reads high rather than low because the PCM input uses a pull-up resistor to the reference rail — making a broken wire produce a P0406 rather than a P0405, which is counterintuitive during diagnosis.
As with P0405, P0406 does not necessarily indicate a mechanically failed EGR valve. The valve may be functioning correctly while the position feedback circuit reports an incorrect high-voltage signal to the ECM. However, if the valve is stuck in the fully-open position and the sensor correctly reports that condition as high voltage (depending on calibration polarity), the code can be triggered by a mechanical valve fault. The ECM's response is typically to disable closed-loop EGR control, substitute a fixed flow value, and illuminate the MIL.
Common causes
Most-frequently reported root causes when P0406 is logged.
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1
Short to the 5V reference voltage on the sensor \"A\" signal wire — insulation damage causing the signal line to contact the reference conductor.
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2
Short to battery voltage (12V) on the sensor \"A\" signal wire.
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3
Open circuit in the sensor signal return (ground) path — with a PCM pull-up reference, a broken return wire reads as a sustained high voltage.
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4
Failed EGR position sensor \"A\" with an internal fault holding the potentiometer output at maximum voltage.
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5
Corroded or bridged connector pins between the signal pin and the reference/supply pin.
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6
EGR valve mechanically stuck in the fully-open position, correctly reporting maximum voltage (fault may be mechanical, not electrical).
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7
Incorrectly spliced or mis-routed wiring repair connecting the signal line to a voltage source.
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8
PCM input circuit failure holding the EGR sensor \"A\" channel high (uncommon — only after all external causes are cleared).
Symptoms drivers notice
How to diagnose P0406
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and retrieve all codes; check whether EGR flow codes (P0401, P0402) or sensor B codes (P0407, P0408) are stored alongside P0406.
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2
With the ignition on and engine off, monitor live EGR sensor \"A\" voltage — a reading consistently at or above 4.5V (or near 5V) confirms the high-circuit condition.
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3
Inspect the sensor connector and wiring harness; with the connector unplugged, measure voltage on the signal wire at the harness side — any voltage present (should be near 0V with connector disconnected on most systems) confirms a short to voltage in the harness.
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4
Verify 5V reference line integrity at the sensor connector; if the reference wire has shorted into the signal wire, both will read incorrectly.
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5
Measure sensor \"A\" signal return (ground) path continuity from the connector to chassis ground — an open return circuit is a common cause of a high-input reading.
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6
Disconnect the sensor and measure the sensor \"A\" potentiometer resistance across signal and ground pins; an open potentiometer element (infinite resistance) that forces the ECM input high via its pull-up resistor confirms a failed sensor.
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7
Inspect the EGR valve mechanically for a stuck-open condition (heavy carbon, seized actuator); if the valve cannot be moved to the closed position, address the mechanical fault first.
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8
If the harness and sensor test serviceable, perform an ECM input-circuit check before condemning the module.
Vehicles where we've handled P0406
Platforms in our catalogue with confirmed P0406 coverage.
Related powertrain codes
- P0400 — Exhaust Gas Recirculation Flow Malfunction
- P0401 — Exhaust Gas Recirculation Flow Insufficient Detected
- P0402 — Exhaust Gas Recirculation Flow Excessive Detected
- P0403 — Exhaust Gas Recirculation Circuit Malfunction
- P0404 — Exhaust Gas Recirculation Circuit Range/Performance
- P0405 — Exhaust Gas Recirculation Sensor A Circuit Low
Frequently asked questions
Why can an open wire cause a high-input code rather than a low-input code?
On systems where the PCM input has an internal pull-up resistor connected to the reference rail, a broken signal wire leaves the input pin floating high through the pull-up, causing the ECM to read a continuous high voltage and set P0406. This is counterintuitive — technicians often expect an open circuit to produce a low reading. Always check for open-circuit conditions (not just shorts) when diagnosing P0406.
Is P0406 more serious than P0405?
Both codes indicate an EGR sensor \"A\" circuit fault and produce similar severity: the EGR system loses closed-loop control and the MIL illuminates. P0406 (high) may cause slightly more immediate idle concerns if the ECM interprets the high voltage as a fully-open valve and aggressively retards fuelling. Neither code constitutes a critical failure, but both should be diagnosed promptly to avoid NOx emissions non-compliance and progressive component wear.
Can the EGR valve be stuck open and cause P0406?
Yes, depending on the sensor calibration polarity. On valves where the sensor outputs maximum voltage when fully open, a mechanically seized valve at the open position will correctly report that condition as a high-voltage signal, which the ECM flags as P0406 because it commanded a different position. Diagnosing mechanical valve movement is an important step before assuming an electrical fault.
Do P0405 and P0406 ever appear together?
Rarely simultaneously on the same sensor, since a single circuit fault drives the signal in one direction. However, if there is an intermittent wiring fault, both codes may appear in the history on alternate drive cycles. If both are stored as pending at the same time, suspect a shared reference or ground circuit fault affecting the sensor's ability to stay within range in either direction.
Disabling P0406 in software
RaceTune can permanently disable P0406 — 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 P0406 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 EDC17C74 verified 2 software versions
- Bosch EDC17C50 verified 1 software version
- Bosch EDC17CP44 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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