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P0406 Generic (SAE)

Exhaust Gas Recirculation Sensor A Circuit High

Updated 2026-10-02 · Auxiliary emission controls

Severity
Medium
Typical repair cost
60 – 550 EUR
System
Powertrain

What it means

P0406 is the counterpart of P0405: the EGR position sensor signal is above the plausible maximum, typically over 4.7-4.9 V, or reports a fully open valve when the ECM has commanded it closed. The usual causes are a sensor ground that has been lost (the signal floats to reference voltage), a signal wire shorted to 5 V or 12 V, or a valve mechanically stuck open. The ECM disables closed-loop EGR control while the code is active.

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Symptoms

  • Check engine light
  • Rough or unstable idle and stalling at stops if the valve really is stuck open
  • Hesitation and poor cold running, exhaust gas diluting the intake at idle
  • Knock under load if the ECM shuts EGR off completely
  • Failed emissions inspection

Common causes

  1. Open sensor ground wire or corroded ground pin at the EGR connector
  2. Signal wire shorted to the 5 V reference or to battery voltage
  3. EGR valve mechanically stuck open by carbon, so the sensor correctly reports open
  4. Position sensor failure, worn track reading high
  5. Water intrusion in the connector bridging pins
  6. ECM input circuit fault (rare)

How to diagnose

  1. Check live EGR position with key on, engine off and valve commanded closed. A reading near 100 percent or 4.5-5 V confirms the high signal. Note whether other 5 V sensors behave normally.
  2. Check sensor ground: with the connector on, measure voltage drop from the ground pin to battery negative; it must be under 0.1 V. Over 0.5 V or floating means an open ground.
  3. Check for a short to voltage: with the sensor unplugged and key on, the harness signal pin should show the ECM pull-up only (usually 0 V or around 5 V depending on design, never 12 V).
  4. Measure the potentiometer: 3-6 kohm total, and verify the signal sweeps from about 0.5 V (closed) to 4.5 V (open) as the valve is moved. Stuck at the high end means sensor or jammed valve.
  5. Command the valve with the scan tool and observe the actuator; also listen for the motor. If the position does not change and the motor runs, the valve is seized; remove and inspect for carbon.
  6. Repair wiring or replace the valve, then run the EGR learn procedure and verify the closed position reads 0-10 percent at idle with stable idle speed.

Frequently asked questions

Why does my engine idle badly with P0406?

If the valve really is stuck open, exhaust gas enters the intake at idle where the engine cannot tolerate it. Cleaning or replacing the valve fixes both the code and the idle.

Will a cheap EGR valve work?

Position sensors on budget valves often read slightly off and set P0405/P0406 again after a few months. Use OE or a reputable brand.

Do I need to reset anything after replacement?

Many manufacturers (VAG, BMW, PSA, Renault) require an EGR basic setting or adaptation with a diagnostic tool so the ECM learns the new closed position.

Related codes

P0405P0404P0403P0402P0401

Nearby codes in this group