P0406 Code: EGR Sensor 'A' Circuit High

Quick Answer

• What it means: The Engine Control Module (ECM) or Powertrain Control Module (PCM) detects that the voltage signal from the Exhaust Gas Recirculation (EGR) Sensor "A" is higher than the manufacturer's specified maximum parameter.
• Severity: Moderate to High. While the vehicle will usually run, ignoring it can lead to engine pinging, failed emissions tests, and potential long-term catalytic converter damage. Get it repaired soon.
• Most common cause: A failed EGR position sensor or Differential Pressure Feedback EGR (DPFE) sensor, or a short-to-power in the sensor's wiring harness.

What the P0406 Code Means

Official Definition: Exhaust Gas Recirculation (EGR) Sensor "A" Circuit High

The Exhaust Gas Recirculation (EGR) system routes a small amount of exhaust gas back into the intake manifold to lower combustion temperatures and reduce oxides of nitrogen (NOx) emissions. To monitor whether the EGR valve is opening and closing correctly, the PCM relies on an EGR position sensor (often built into the valve itself) or a DPFE sensor.

Code P0406 triggers when the PCM receives a voltage signal from this "A" sensor circuit that is continuously higher than normal. Most EGR position sensors are 3-wire potentiometers operating on a 5-volt reference. When the valve is closed, the signal voltage should be low (usually around 0.4 to 0.8 volts). When it opens, the voltage sweeps up to around 4.5 volts. If the PCM sees a constant 4.6 to 5.0+ volts regardless of commanded EGR position, it assumes there is a circuit fault or the sensor has failed, and it sets P0406.

Common Symptoms

If your vehicle has triggered a P0406 code, you will likely experience one or more of the following:

• Illuminated Check Engine Light (MIL): The most guaranteed symptom.
• Engine Pinging or Knocking: If the EGR system is disabled or the valve is stuck closed due to the fault, combustion temperatures will rise, leading to pre-ignition (pinging) on acceleration.
• Rough Idle or Stalling: If the high voltage is caused by an EGR valve that is physically stuck wide open, exhaust gases will flood the intake at idle, starving the engine of oxygen.
• Reduced Fuel Economy: An inefficient combustion process will lower your MPG.
• Failed Emissions Test: Because the EGR system directly controls NOx emissions, any fault here will result in an automatic emissions failure.

Common Causes

Ranked from most to least likely, the culprits behind P0406 include:

1. Failed EGR Position Sensor / DPFE Sensor: The internal potentiometer tracks wear out, or the sensor internally shorts out, constantly sending a 5V signal back to the PCM.
2. Wiring Issues (Short to Voltage): The signal wire is chafed and touching the 5-volt reference wire or battery voltage wire, driving the signal artificially high.
3. Wiring Issues (Open Ground): If the sensor loses its ground connection, the circuit voltage will default to the 5V reference high limit.
4. Carbon-Packed EGR Valve: Severe carbon buildup can cause the valve to physically jam open, pushing the position sensor to its maximum limit.
5. Faulty PCM: Very rare, but an internal short inside the engine computer can cause it to misinterpret the EGR sensor data.

How a Technician Diagnoses It

An ASE-certified technician will follow a logical, step-by-step diagnostic path to isolate the exact cause of the P0406 code rather than firing the parts cannon at it.

1. Scan Tool Data & Freeze Frame: We start by connecting a professional-level scan tool to pull all DTCs and read the freeze-frame data to see exactly what the engine conditions (RPM, load, temp) were when the code set.
2. Live Data Monitoring (PID Check): We pull up the "EGR Position Sensor Voltage" PID on the scanner. We want to see what the PCM is seeing. If it reads a steady 4.8V or 5.0V with the engine off, we know we have a hard fault.
3. Sensor Disconnect Test: We unplug the EGR sensor connector. If the live data voltage drops to 0V (or a low default value), the wiring is likely fine, and the sensor itself is shorted internally. If the voltage remains high (5V) even when unplugged, there is a short to voltage in the wiring harness between the sensor and the PCM.
4. Check Power and Ground: Using a digital multimeter (DMM), we probe the connector for a solid 5-volt reference signal and a solid ground. A missing ground is a very common cause of an artificially high sensor reading.
5. Manual Valve Actuation: If the electronics check out, we remove the EGR valve and check for excessive carbon blockage. We may apply vacuum (or command it bi-directionally on electronic valves) to see if the pintle moves freely. If it is jammed wide open by a chunk of carbon, it will max out the position sensor voltage.

Typical Repair Cost

Depending on the underlying cause and the make/model of your vehicle, repair costs for a P0406 code typically fall into these ranges (including parts and labor):

• EGR Position Sensor / DPFE Sensor Replacement: $100 – $250
• EGR Valve Replacement (if sensor is integrated): $250 – $450
• Wiring/Connector Repair: $100 – $200 (mostly diagnostic and labor time)
• EGR System Decarbonization/Cleaning: $150 – $300

Can I Keep Driving with this Code?

You can technically drive with a P0406 code, but it is not recommended for an extended period. If the EGR system is disabled, the elevated combustion temperatures can cause severe engine knocking (detonation), which is highly destructive to pistons and valves over time. If the valve is stuck open, the vehicle may be miserable to drive, stalling at stoplights and running terribly at low speeds. It is best to have this diagnosed and repaired as soon as possible to prevent collateral engine damage.

Related Codes

Technicians often see P0406 accompanied by other EGR-related DTCs. Some nearby codes include:

• P0404: Exhaust Gas Recirculation Control Circuit Range/Performance
• P0405: Exhaust Gas Recirculation Sensor "A" Circuit Low
• P0400: Exhaust Gas Recirculation Flow Malfunction
• P1404: IAT - B Circuit Malfunction / Exhaust Gas Recirculation (EGR) Closed Position Performance

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