Exhaust Gas Recirculation Sensor A Circuit Low
The ECU detects that the EGR sensor voltage is reading too low, like a dimmer switch stuck on a very dark setting. This means the sensor isn't communicating properly with the engine control computer about exhaust gas flow.
The ECM monitors the analog voltage signal from the EGR position sensor, which indicates how open or closed the EGR valve is. When voltage drops below approximately 0.5V at idle, the ECM registers a low circuit fault. This threshold protects against wiring shorts, sensor failures, or open circuits in the EGR sensor circuit.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| EGR Sensor Voltage | 0.5V to 4.5V | Below 0.5V |
| EGR Valve Position | 5% to 95% range | Below 5% (stuck closed) |
Code P0406 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0406 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.