Faulty EGR valve position sensor or integrated sensor inside the EGR valve (often carbon fouled); Damaged wiring or poor connector contact on the sensor signal, reference or ground circuit (open ground or short to supply); Corroded or loose terminals at the EGR valve or sensor connector; EGR valve stuck open or closed by carbon build-up, giving a signal outside the expected range; Sensor reference voltage fault from another shared sensor circuit or a supply problem; Faulty engine control unit.

P0406

EGR Sensor A Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
4: AreaAuxiliary emission controls
06: FaultThe specific fault in that area

What it means

P0406 is the generic code for an exhaust gas recirculation (EGR) sensor A circuit high fault. The engine control unit has seen the signal voltage from the EGR position or flow sensor stay above the expected range, which usually points to a short to voltage, an open circuit in the signal or ground wire, or a faulty sensor. Which sensor is 'A' and how it is wired depends on the vehicle and engine.

Symptoms

Engine warning light on
Rough or unstable idle
Hesitation or poor pulling under load
Slightly higher fuel consumption
Pinking or knocking under load on some engines
Failed emissions test

Possible causes

Faulty EGR valve position sensor or integrated sensor inside the EGR valve (often carbon fouled)likely
Damaged wiring or poor connector contact on the sensor signal, reference or ground circuit (open ground or short to supply)likely
Corroded or loose terminals at the EGR valve or sensor connectorpossible
EGR valve stuck open or closed by carbon build-up, giving a signal outside the expected rangepossible
Sensor reference voltage fault from another shared sensor circuit or a supply problempossible
Faulty engine control unitrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including engine load, temperature and speed when the fault set
Check for related codes such as other EGR, MAP or sensor supply faults that could point to a shared reference or ground problem
Visually inspect the EGR valve, sensor, harness and connector for chafing, heat damage, water ingress, corrosion and bent or pushed-back pins
View live data for the EGR position or sensor voltage and compare it with the commanded value at key on, idle and when the valve is commanded
Check the sensor reference supply, signal and ground at the connector with a multimeter against the manufacturer's specification
Check the wiring for continuity and for shorts to voltage between the connector and the control unit
Remove and inspect the EGR valve for carbon build-up or a sticking valve, following the model's procedure
Check for a relevant software update or technical bulletin for your model if the wiring and components test good

How to fix

Repair or replace damaged wiring and renew faulty or corroded connector terminals
Clean or replace the EGR valve or sensor if it is fouled, sticking or giving an out-of-range reading
Replace the EGR valve assembly where the sensor is not available separately
Rectify any shared sensor supply or ground fault found during testing
Update the control unit software only if the manufacturer lists an outdated calibration as a known cause for this fault, and replace the control unit only after all other causes are ruled out

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but book it in soon as driveability may worsen and emissions will be affected. Avoid heavy loads and long trips if the engine runs roughly or knocks.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.