P0406

Universal

Exhaust Gas Recirculation Sensor A Circuit High

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Medium
Medium Severity

The engine computer has detected an electrical signal from the EGR Sensor A circuit that is higher than expected. The EGR system recirculates some exhaust gas to help control emissions, and its sensor provides feedback to the computer. This code identifies a detected circuit condition; it does not prove that the sensor or EGR valve needs replacement. Depending on the vehicle and the underlying cause, the warning light may be the only symptom, or the engine may run poorly or produce excess emissions.

Report an error with this code

⚠️ Before You Begin (Safety First)

If the engine stalls, loses power, or runs too poorly to control safely, stop driving and arrange an inspection. Otherwise, avoid hard driving and have the fault diagnosed soon, especially if the warning light flashes or other serious symptoms appear.

Common Symptoms

  • The check-engine light may be on, with no noticeable change in how the vehicle runs.
  • Rough idle, hesitation, or stalling may occur if the underlying EGR fault disrupts exhaust-gas flow or engine operation.
  • Reduced performance may occur if the engine computer changes EGR operation in response to the detected fault.
  • Emissions may increase if the EGR system cannot operate or be monitored correctly.

Common Causes

  • A damaged wire or connector may allow the sensor signal to become electrically high, including through an unintended connection to a voltage source.
  • A poor electrical connection or ground can distort the signal reported by the EGR Sensor A circuit.
  • The sensor or an EGR component that incorporates it may produce an abnormally high signal.
  • A fault elsewhere in the circuit or, less commonly, in the engine computer may cause the detected condition.

Diagnostic Steps

  1. Record any other diagnostic codes and available freeze-frame information before clearing codes; related faults can help identify whether the issue is isolated or part of a wider electrical or EGR problem.
  2. With the engine off and cool, visually inspect accessible EGR-related wiring and connectors for loose connections, damaged insulation, corrosion, or contamination. Do not touch hot exhaust parts.
  3. If you have a scan tool that supports the vehicle, review available EGR sensor data and note whether it appears implausible or changes when the system operates; use vehicle-specific information to interpret the readings.
  4. Avoid replacing parts based only on this code. If electrical testing or access to the sensor is needed, have a qualified technician follow the vehicle-specific procedure.

When to Call a Professional

A technician can compare EGR-related scan data with the expected operation for the specific vehicle, inspect the circuit and connector, and test the sensor signal, power, and ground using model-specific service information. They can also check whether the EGR system responds appropriately to commanded operation before deciding whether a sensor, wiring repair, or another fix is needed.

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Commonly Asked Questions

Does P0406 prove the EGR sensor is faulty?

No. It reports a high electrical signal in the EGR Sensor A circuit. Wiring, connections, grounding, the sensor, or another circuit fault may be responsible.

Can I keep driving with P0406?

If the vehicle runs normally, a short trip for diagnosis may be reasonable, but arrange an inspection soon. Stop driving if it stalls, loses power, or becomes difficult to control.

Does “circuit high” mean the EGR valve is stuck open?

Not necessarily. “Circuit high” describes the detected electrical signal, not a confirmed valve position or mechanical failure.

Why might there be no symptoms?

The engine computer may detect an out-of-range signal before the fault noticeably affects engine operation, or the fault may have little immediate effect under the vehicle's current operating conditions.

Browse Available Code Guides

Previous and next available guides in database record order, across makes. Adjacent entries may share a code number and do not necessarily describe related faults.