P0405 P0xxx · Powertrain (P) Medium severity
Exhaust Gas Recirculation Sensor A Circuit Low
The EGR position sensor voltage is below its plausible minimum — a short to earth, an open reference, or a dead sensor.
Severity guidance: Medium — drivability or emissions affected, book it in this week
Symptoms you will notice
- EGR disabled
- Knock under load
- Reduced power
- Failed emissions test
Common causes
Ordered from most to least likely — start at the top.
- 1 Short to earth in the signal wire
- 2 Failed EGR position sensor
- 3 Missing 5 V reference
- 4 Water ingress in the connector
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Check for 5 V reference and a good earth at the sensor connector.
- ✓ Back-probe the signal wire; it should sweep smoothly as the valve moves.
- ✓ Disconnect the sensor and see whether the voltage returns to reference — that separates sensor from wiring.
Repair options
- Replace the EGR valve assembly
- Repair the wiring
- Clean and seal the connector
P0405 — frequently asked questions
Can I repair just the sensor? +
On most modern valves the position sensor is integral — you replace the whole valve.
Why does the ECU disable EGR? +
A failsafe: it cannot trust the feedback, so it commands the valve closed to avoid stalling the engine.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.