P0406–P0408 EGR Sensor A Circuit High
The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.
Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P0406 | EGR Sensor A Circuit High | Investigate soon |
P0408 | EGR Sensor B Circuit High | Investigate soon |
The definition
P0406: EGR Sensor A Circuit High.
P0406 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.
What to check first
Cheapest and fastest checks first:
- Unplug the sensor and check whether the signal falls. If it stays high, suspect the wiring rather than the sensor.
- Check the earth path - a missing earth makes a sensor read high.
What actually causes this on a VAG
The EGR valve feeds a metered amount of exhaust gas back into the engine to keep combustion temperatures and NOx down, and a position sensor - a potentiometer, built into the valve itself on most modern VAG engines - tells the computer where the valve actually is. "Circuit High" is an electrical complaint: the voltage on that sensor's signal wire is sitting above the valid range the ECU will accept, whatever the valve happens to be doing at the time. That distinction matters, because a sooted-up, sticking or lazy EGR valve has its own separate codes on VAG - insufficient flow, stuck open, stuck closed, and sensor range/performance are all defined individually. This code points first at wiring, not at carbon.
Most likely causes, in order
- A lost sensor earth or a short to a supply at the valve plug. This is the classic reason a position signal pegs at the top of its range: the sensor's earth path goes open circuit, or the signal wire touches a supply voltage, and the reading rises to the ceiling. The connector sits in a hot, dirty spot. Check it for corrosion, spread or pushed-back pins, melted insulation and a loose latch before anything is unbolted.
- Damaged wiring between the valve and the ECU. The loom runs through a hot part of the engine bay and can chafe on a bracket or heat shield, or simply harden and crack with age. Follow it looking for rubbed or brittle insulation and any previous repair joint, and wiggle it section by section while someone watches the live EGR position value on a diagnostic tool. A reading that jumps to full scale as you move one particular section has just named the fault.
- A failed position sensor inside the valve. The potentiometer is normally part of the valve assembly and is not sold separately, so this means replacing the valve. It is a reasonable conclusion only once the connector and the loom have been proved good, because the wiring faults above produce exactly the same reading for a fraction of the cost.
- A valve jammed hard against a stop by carbon. Worth a look, but keep it in proportion. Heavy carbon can drive the position feedback out of range, yet on VAG a coked valve far more usually sets a flow, stuck-open, stuck-closed or range/performance code rather than this one. Do not strip the inlet on the strength of this code alone.
Worth knowing: After an EGR valve is cleaned or replaced, a VAG-capable tool needs to run the basic settings routine so the ECU relearns the valve's end stops. Skip that step and a fault very often comes straight back, which is how people end up buying a second valve they never needed. And on this particular code, check the wiring before you buy the first one.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
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Common questions
What does P0406 mean?
P0406 means EGR Sensor A Circuit High. The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.
Can I drive with P0406?
The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.