P0406
EGR Sensor A Circuit High
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The P0406 code indicates that your vehicle's powertrain control module (PCM) has detected a higher-than-expected voltage in the Exhaust Gas Recirculation (EGR) Sensor A circuit. The EGR system reduces harmful nitrogen oxide (NOx) emissions by recirculating a portion of exhaust gases back into the intake manifold, lowering combustion temperatures. The position sensor monitors how much the EGR valve is opening and closing, and a high-circuit reading tells the PCM that something is outside the normal operating range.
In most cases, P0406 is caused by a faulty EGR position sensor, damaged wiring, or a corroded connector. Carbon buildup inside the EGR valve is also a frequent contributor, particularly on higher-mileage vehicles. The repair is generally straightforward and DIY-friendly — a basic inspection of the wiring harness and connector followed by sensor or valve replacement is all that is typically required. Cleaning the EGR valve passages with carburetor cleaner is a worthwhile first step before replacing parts.
While P0406 is not an emergency, it should be addressed within the week to avoid worsening fuel economy, rough idling, and a failed emissions inspection. Driving with a stuck-open EGR valve for an extended period can also lead to engine hesitation and, in some cases, increased wear on internal components. Use an OBD2 scanner to check for any accompanying codes such as P0400, P0401, or P0405, as these can help pinpoint whether the issue is the sensor, the valve, or the wiring circuit.
Severity
Symptoms
- •Check Engine Light illuminated
- •Rough idle or engine stumbling
- •Reduced fuel economy
- •Failed emissions test
- •Engine hesitation or stumbling during acceleration
- •Possible black smoke from exhaust
Likely Causes
The EGR position sensor itself has failed or developed an internal short, causing it to send a voltage signal outside the expected range. This is the most common cause and often requires sensor replacement.
Damaged, corroded, or shorted wiring in the EGR sensor circuit can cause the PCM to read an abnormally high voltage signal. Inspect the harness and connector for chafing, corrosion, or broken wires.
Excessive carbon deposits can cause the EGR valve to stick open or restrict movement, leading the sensor to report a high-circuit condition. Cleaning the valve and passages may resolve the issue.
The EGR valve itself may have failed mechanically or electrically, causing the sensor to report an out-of-range signal. If the valve is stuck in the open position, it can also cause drivability problems.
In rare cases the powertrain control module may misinterpret or fail to process the EGR sensor signal correctly. This is typically only diagnosed after all other causes have been ruled out.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the EGR position sensor
- Inspect and repair damaged or corroded wiring and connectors in the EGR sensor circuit
- Clean carbon deposits from EGR valve and passages
- Replace the EGR valve assembly
- Clear codes and retest after repair to confirm resolution
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