P0406
PowertrainEGR Sensor A Circuit High
Most people first notice this as a warning light along with a slightly rough engine, a bit of pinging under load, or the odd surge at a steady motorway cruise. Underneath it, the EGR valve has a position sensor that tells the ECU how far open the valve is, and that sensor is sending back a voltage that's too high. The ECU sees the high signal, decides it can't trust the EGR position, and flags P0406. Nine times in ten on UK diesels it comes back to a carboned-up valve or a tired sensor rather than anything dramatic.
ⓘ Information only. This page provides general educational information about fault code P0406. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0406 mean?
P0406 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: EGR Sensor A Circuit High.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, usually steady rather than flashing
- • A bit of pinging or knock under acceleration, particularly going up hills or loaded
- • Rough idle, sometimes a stall when you come to a stop
- • Fuel economy down by a few mpg, more obvious on shorter runs
- • Hesitation or a flat spot when you put your foot down
- • Occasional surging at a constant speed on the motorway
Possible causes
Causes commonly associated with P0406, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. EGR position sensor itself reporting a falsely high voltage, common once the sensor has aged or been baked by exhaust heat over the years
- 2. Wiring or connector trouble in the sensor circuit, corroded pins and chafed insulation are typical near a hot exhaust
- 3. Carbon clogging the EGR valve so the sensor reads a position that doesn't match what the ECU expects, very common on high-mileage VW/Audi 1.9 and 2.0 TDI engines and the PSA 1.6 HDi family
- 4. EGR valve mechanically stuck or seized, often because of that same carbon
- 5. A dead short in the signal wire, either to battery voltage or the 5V reference, which pins the reading high
- 6. Faulty ECM, but this is rare and only worth chasing once everything else checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull the freeze frame data and note what the engine was doing when it logged, idle, load, temperature, it tells you a lot about whether you're chasing a sensor or a sticking valve
- 2. Unplug the EGR sensor connector and look hard at the pins for green corrosion, melted plastic, or a pin that's backed out. Give the wiring a wiggle and watch the live voltage
- 3. Back-probe the sensor signal wire with a multimeter. At idle you'd expect roughly 0.5 to 1.5V. If it's sat up at 4.0 to 4.5V or higher with the valve closed, you've found your high signal
- 4. Pull the valve and check it for carbon. A diesel EGR at 90,000-plus miles is often half blocked with soot, and that throws the position reading off
- 5. Check the signal wire for a short to battery or reference voltage if the sensor itself tests fine and the connection is clean
- 6. Sensor good, wiring good, valve clean and moving freely? At that point the valve assembly or, rarely, the ECM is what's left.
Common questions about P0406
Can I sort this out in my own driveway? +
Often, yes, if it's the usual cause. On a lot of diesels the valve is held on with two or three bolts and comes off in twenty minutes. Soak the carbon with EGR cleaner, scrub it out, check the sensor connector for corrosion and clean it with contact cleaner. That fixes a fair share of P0406s for the price of a can of cleaner. Where it gets beyond DIY is if the sensor is integrated into the valve body and the whole unit needs replacing, or if you're chasing a wiring short, which means a meter and patience. Budget around £50 to £150 for a sensor or valve assembly on common cars, more on a dealer-only part.
If I clear the code, will it stay gone? +
Depends entirely on why it set. If it tripped on a one-off glitch or a loose connector you've since reseated, clearing it might be the end of it. But if the valve is gummed up with carbon or the sensor is on its way out, the light will be back within a handful of drive cycles, sometimes the same day. Clearing the code without fixing the cause just hides the problem until the ECU rechecks the sensor.
What's the harm if I just leave it? +
It won't strand you, the car will keep running, but you're not getting away clean. EGR faults often nudge the engine into a low-power limp mode and knock a few mpg off your fuel. Worse, with the EGR not metering properly you can get pinging under load, and sustained knock is what cracks pistons and ringlands over time on a diesel. There's also the MOT to think about, an illuminated warning light at test time can fail you on its own, and a genuine emissions fault will fail the smoke test. Cheaper to deal with it while it's just a sensor or a clean.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →