P0405 and P0406: EGR position sensor A low and high

An EGR valve without position feedback is an open loop, and the ECU does not tolerate open loops on emissions hardware. P0405 and P0406 say the position signal has gone outside its plausible voltage window.

Code	Meaning
P0404	EGR position sensor range/performance
P0405	Circuit low input
P0406	Circuit high input

Detail on the range/performance version: P0404 EGR position sensor diagnostics.

Circuit codes versus performance codes

P0405 and P0406 are electrical. The voltage is outside the range a working sensor can produce — below the minimum or above the maximum.

P0404 is behavioural. The voltage is plausible but the movement does not match what was commanded.

That distinction directs the work. Electrical codes mean check wiring, connector and reference voltage. Behavioural codes mean check carbon.

What a healthy signal looks like

Most EGR position sensors are potentiometric — a wiper on a resistive track, powered by the 5 V reference.

Valve state	Typical signal
Fully closed	Near the bottom of the range, but not at zero
Fully open	Near the top, but not at maximum
Moving	Smooth sweep between the two

Neither extreme should ever be reached. A design that reads exactly 0 V when closed cannot distinguish “closed” from “wire broken”, so manufacturers keep the working range inside the extremes deliberately.

Which means: a reading of 0.0 V or 5.0 V is a circuit fault, not a valve position.

Causes

The connector

First, always. Soot, heat and vibration in the one place they all coincide. Look for green oxide and for pins that have lost tension.

The 5 V reference

Shared with other sensors. A short in the EGR sensor pulls the reference down and sets codes on several unrelated sensors at once.

If P0405 arrives with MAP, throttle and rail pressure codes, do not diagnose four sensors. Diagnose the reference.

Wiring

The harness runs across the hottest part of the engine. Insulation hardens, cracks, and eventually shorts to the manifold.

The sensor

On integrated valves, the sensor is part of the valve assembly and cannot be replaced separately. That makes the cheap checks above worth doing thoroughly first.

A seized valve

A valve stuck at one extreme can push the sensor beyond its normal range. In that case the electrical code is a symptom of carbon after all.

Distinguishing sensor from valve

The most useful check on the whole system:

Log commanded EGR position and actual EGR position together during a drive.

Pattern	Meaning
No actual value at all	Sensor or circuit — P0405/P0406
Actual pinned at an extreme	Circuit fault or seized valve
Actual moves but lags	Carbon, valve sticking
Actual tracks commanded	Feedback healthy — look at flow instead

Once actual position tracks commanded position, the sensor is exonerated and the investigation moves to flow: P0400 and P0403 EGR flow.

Symptoms

• Rough idle, especially on a diesel.
• Reduced power, sometimes limp mode.
• Increased smoke or a change in exhaust smell.
• Emissions failure on NOx.
• On some vehicles, a DPF that regenerates more often because combustion changed: DPF light on, what to do.

The whole-system view

An EGR fault rarely stands alone on a high-mileage diesel. Carbon that blocked the valve also coated the intake, and the intake runner flaps are in the same tract: P2004 to P2010 intake manifold runner control.

If you are removing the intake to clean the EGR passages, inspect the runner flaps while you are there. Doing the job twice is the expensive outcome.

Step by step

1. Read all codes. A crowd of sensor codes means the reference, not the sensors.
2. Inspect and clean the connector.
3. Check the 5 V reference with ignition on.
4. Log commanded versus actual position.
5. Check for carbon if actual moves but lags.
6. Replace the valve assembly only when the electrical path is proven good.

Pair an ELM327 adapter, open the free tier of the app and watch actual EGR position during a warm drive. A value that never appears and a value that appears but lags are two different repairs.