P0405
PowertrainEGR Sensor A Circuit Low
The EGR position sensor 'A' circuit is reading low voltage. The sensor that reports the EGR valve's position back to the ECU isn't producing the expected signal, suggesting either a sensor fault or a wiring issue.
ⓘ Information only. This page provides general educational information about fault code P0405. 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 P0405 mean?
P0405 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: EGR Sensor A Circuit Low.
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
- • Rough idle, particularly noticeable at low revs
- • Hesitation on acceleration, especially when accelerating gently from low speeds
- • Possibly increased emissions, possible MOT smoke test failure on diesels
- • On some cars, the EGR may default to closed which actually improves drivability briefly while the code is set
Possible causes
Causes commonly associated with P0405, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed EGR position sensor (separate from the valve on some cars)
- 2. Wiring damaged or shorted to ground, often where the loom passes near the exhaust manifold or EGR cooler
- 3. Connector contaminated with carbon, oil, or moisture from the EGR system itself
- 4. Less commonly: failed EGR valve unit where the sensor is integrated
- 5. On VAG TDIs particularly: the sensor connector is sometimes worn from heat-soak failure
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. Visual inspection of the EGR valve and sensor connector, look for carbon, oil, moisture, or damaged pins
- 2. Test sensor electrically against spec, with the valve closed the signal should be one value, with it commanded open another value
- 3. Check wiring continuity from connector to ECU
- 4. If the sensor is integrated into the valve assembly, the whole assembly may need replacing
- 5. Some manufacturers offer a service procedure to clean the sensor port; worth checking the workshop manual
Common questions about P0405
I cleaned the EGR but the code is still there +
If the position sensor is integrated into the valve assembly (common on diesel EGRs), cleaning the valve doesn't help the sensor. Some cars need full EGR assembly replacement; some have a separate sensor that can be replaced cheaper. Check whether your specific make has a standalone sensor before assuming the whole valve needs replacing, the difference is £30 vs £400.
Why does the EGR system get so blocked? +
Diesel EGR systems route hot, sooty exhaust gas back into the intake. Over time, soot mixes with oil mist from the crankcase ventilation system and forms a hard, tar-like residue that builds up in the EGR valve, intake manifold, and around any sensors in the EGR pathway. Cars used mostly for short urban journeys (low exhaust temperatures, more soot) build up this gunk fastest. Motorway-heavy use significantly slows the buildup.
Can I 'delete' or block off the EGR? +
Tampering with emissions equipment makes the car illegal for road use and an automatic MOT failure post-2018. Some workshops offer EGR delete services, but they're not legal for road cars in the UK. The proper fix is clean or replace.
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 →