P0405–P0407 EGR Sensor A Circuit Low
The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.
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 |
|---|---|---|
P0405 | EGR Sensor A Circuit Low | Investigate soon |
P0407 | EGR Sensor B Circuit Low | Investigate soon |
The definition
P0405: EGR Sensor A Circuit Low.
P0405 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. If the reading jumps to full scale, the wiring is fine and the sensor has failed low.
- Check the sensor's 5V supply and its earth before condemning the sensor itself.
What actually causes this on a VAG
The EGR valve on most VAG engines has a position sensor built into it, so the ECU can check the valve actually went where it was told. P0405 means that feedback signal is sitting below its normal range - typically stuck near the bottom of the scale, which points at an open circuit, a short to earth, or a sensor that has failed low. On most VAG engines that sensor isn't a separate part; it lives inside the valve unit. So a "sensor" code usually turns into either a wiring repair or a valve.
Most likely causes, in order
- Wiring or connector fault. A chafed wire, a corroded pin or water sitting in the plug will hold the signal at the bottom of its range. Pay particular attention to where the EGR loom passes close to hot metal or is tied down against a bracket. With the ignition on, watch the reported EGR position: a dead-flat value that never moves, or one that flickers when you wiggle the connector, points straight here. This is the cheapest thing on the list, so rule it out first.
- Failed position sensor inside the EGR valve. The sensor lives in constant heat and eventually fails; on the older potentiometer-type sensors the track can wear where the valve spends most of its life, so the signal drops out over part of the travel. Sweep the valve in an output test and watch for the reading collapsing to the bottom of the scale part way through. On most VAG valves the sensor isn't sold separately, so this means the valve.
- Lost sensor supply or earth from the ECU. The low-voltage reference that feeds this sensor is normally shared with other sensors, so if it has been lost you will often see several apparently unrelated sensor faults logged at the same moment. Read the whole fault memory before deciding anything.
Worth knowing: Read every fault in the memory, not just this one, and look at the timestamps. If other sensors logged codes at the same moment, you're chasing a shared wire, plug or earth rather than the EGR valve - and the valve is by far the most expensive of the three things this could be.
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 P0405 mean?
P0405 means EGR Sensor A Circuit Low. The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.
Can I drive with P0405?
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.