What it means
Exhaust Gas Recirculation Sensor "A" Circuit High: the signal voltage is above the expected range. Without valid EGR feedback the ECU disables exhaust gas recirculation and may set further EGR codes.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
EGR sensor A provides the module with valve position or other EGR feedback, depending on the vehicle. P0406 sets when the sensor A signal rises above the expected electrical range, leaving the module unable to trust that feedback for EGR control.
Common causes
Most likely first.
- Sensor circuit shorted to supply
- EGR position sensor failed
- Connector pins corroded - high contact resistance
Symptoms
- Check engine light on
- Rough idle or hesitation
- Reduced engine performance
- Increased emissions
How to diagnose it
- Save freeze-frame data and compare the sensor A signal with commanded EGR position and other available live data.
- With the ignition off, inspect the sensor A wiring and connector for damage, corrosion, loose pins, or contact with a power feed.
- Check the sensor reference, ground, and signal circuits using the manufacturer's wiring information.
- Measure the sensor A signal under the recorded fault conditions and test for a short to supply or a high-resistance connection.
- If the wiring checks pass, test EGR sensor A through its operating range before considering replacement.
Common fixes
- Repair a sensor A signal wire shorted to supply.
- Clean or repair corroded, loose, or damaged connector terminals.
- Repair a faulty reference or ground connection if circuit tests identify it.
- Replace EGR sensor A if it continues to produce an above-range signal on a sound circuit.
Can I keep driving?
If the engine runs normally, a short, gentle trip is generally possible, but avoid driving if it stalls or loses power. Invalid EGR feedback can disable proper EGR control, increase emissions, and lead to additional EGR codes.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.