Exhaust Gas Recirculation Sensor A Circuit High
Diagnostic trouble code P0406 indicates that the engine control module (ECM) has detected a voltage signal from the Exhaust Gas Recirculation (EGR) position sensor 'A' circuit that exceeds the maximum expected threshold. This high voltage condition typically implies an open circuit, a short to power, or a failed sensor within the EGR system.
While not an immediate safety hazard, driving with this code can lead to poor engine performance, increased emissions, and potential engine knocking. It should be diagnosed and repaired soon to prevent long-term engine or catalytic converter damage.
Common symptoms
- Illuminated Check Engine Light
- Rough or unstable engine idle
- Engine pinging or knocking during acceleration
- Engine stalling at low speeds
- Decreased fuel efficiency
Likely causes
- Faulty EGR valve position sensor (often integrated into the EGR valve)
- Corroded, damaged, or loose electrical connector at the EGR sensor
- Wiring issue such as an open circuit or short to power in the sensor signal wire
- Carbon buildup causing the EGR valve to stick open
- Faulty Powertrain Control Module (PCM) (rare)
What to check before replacing parts
Visually inspect the EGR sensor wiring harness and connector.
Look for: Look for broken wires, melted insulation, or corroded pins that could cause an open circuit or short to power.
Monitor the EGR position sensor voltage using a scan tool.
Look for: Observe the live data to see if the voltage is stuck at or near 5 volts, which confirms the circuit high condition.
Test the 5-volt reference and ground circuits at the EGR sensor connector.
Look for: Using a multimeter, verify a steady 5V reference and a solid ground; a missing ground can cause the signal voltage to read high.
Check the signal wire for a short to voltage.
Look for: Disconnect the sensor and the PCM, then measure for voltage on the signal wire; any voltage present indicates a short to power in the harness.
Remove and inspect the EGR valve for severe carbon buildup.
Look for: Check if carbon deposits are physically jamming the valve pintle, which can sometimes force the position sensor into an out-of-range state.
This page explains the generic P0406 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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