Emissions

P0406 — EGR Position Sensor A Circuit High

Severity: medium Safe to drive

What P0406 means

The engine control unit monitors the signal voltage from the EGR valve position sensor, which normally changes within a set range as the valve opens and closes. With P0406, the signal went above the allowed maximum. This points to a failed sensor, a signal wire shorted to voltage, or a wiring fault. The ECU can no longer control exhaust gas flow accurately and usually limits or disables EGR.

Symptoms

  • Check engine light on
  • Rough idle
  • Hesitation or surging under acceleration
  • Occasional stalling at idle
  • Worse fuel economy
  • Reduced performance
  • Emissions test failure

Possible causes

  1. Failed EGR valve position sensor (often built into the valve)
  2. Damaged wiring, corroded terminals or a bad connector in the EGR circuit
  3. Signal wire shorted to battery voltage or the 5 V reference
  4. Carbon buildup or a sticking EGR valve and passages
  5. Open ground circuit to the sensor
  6. Rarely, an ECU fault or software problem

How to diagnose

  1. Read all codes and freeze frame data to see when the fault set
  2. Check battery and charging system voltage
  3. Inspect the EGR valve connector and harness for chafing, moisture, corrosion or heat damage
  4. Compare commanded and actual EGR position in live data while commanding the valve with a scan tool
  5. Use a multimeter to check the 5 V reference, ground and signal at the connector
  6. Test the signal wire for a short to voltage and for an open circuit back to the ECU
  7. Remove the valve and inspect for carbon buildup and sticking, and check the passages
  8. If wiring and valve test good, check the ECU and look for software updates

Common fixes

  • Replace the EGR valve or its position sensor if faulty
  • Repair damaged wiring, replace the connector or clean the terminals
  • Clean the EGR valve and passages of carbon
  • Update the ECU software, or replace the ECU if confirmed faulty
  • Clear codes after repair and run a relearn procedure if the vehicle requires one

Questions and answers

Can I drive with a P0406 code?

Usually yes, for a short time. Emissions will go up, and a valve stuck open can cause stalling, so repair it soon.

Can I just disable or block off the EGR valve?

That doesn't fix the cause of the code and breaks emissions rules. The ECU may still log the fault or run in a limited mode, so it's better to find and fix the actual problem.

Will cleaning the EGR valve clear P0406?

Only if carbon buildup or sticking caused it. If the position sensor or wiring is at fault, cleaning won't help, so test the circuit and signal first.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.