OBD Code P0406: Exhaust Gas Recirculation Sensor A Circuit Low

Q: What does OBD code P0406 mean?

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.

Q: What are the symptoms of fault code P0406?

Check Engine Light illuminated, Rough idle or engine hesitation, Increased emissions or failed emissions test

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What This Actually Means

In plain language — no jargon

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.

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Symptoms You May Notice

3 known symptoms for this code

Check Engine Light illuminated

Rough idle or engine hesitation

Increased emissions or failed emissions test

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Embedded Systems Insight

What the ECU/ECM is actually computing

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.

Voltage & Parameter Thresholds

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)

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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex

1

EGR sensor connector

Inspect and reseat the connector at the EGR sensor for corrosion or poor contact.

2

EGR sensor wiring harness

Check for damaged, pinched, or corroded wires between sensor and ECM; repair or replace damaged sections.

3

EGR position sensor

Replace the sensor if voltage remains low after connector and wiring checks pass.